Dressing salad on Shabbos

Salting salad on Shabbos:[1]

All foods which salt helps to change its natural state[2], [such as] to soften[3] it or harden[4] it or remove its bitterness and other [changes] of the like, which includes all foods which are commonly pickled, may only be salted one piece at a time[5], and must be eaten immediately.[6] It is thus forbidden to salt another piece prior to eating the first piece, as doing so delays the eating of the first piece. Amongst foods included in the above category are: radish[7] or onion or garlic[8], beans and lentils[9] that were cooked in their peels, cucumbers, lettuce. Accordingly, it would be forbidden to salt a salad on Shabbos being that doing so transgresses both of the conditions, as it involves salting more than one piece of food at a time and one does not eat the salted food immediately. There is however one method available in which it is permitted to salt a salad on Shabbos, and that is through adding oil or vinegar to the food either before, or immediately after the salting. If one adds oil or vinegar to the food immediately after salting, then it is permitted to salt even many pieces at the same time and they may even be eaten later on, on Shabbos, although one must do so in close proximity to the meal.[10]

Background:

The above summary is based on a dispute in Rishonim/Poskim that is recorded in Admur and follows the final ruling of Admur as explained in Achronim.

First Opinion:[11]

Not to salt more than one piece at a time: Any item which salt helps to change its natural state[12], [such as] to soften[13] it or harden[14] it or remove its bitterness and other [changes] of the like, such as for example radish[15] or onion or garlic[16] and the like of other spicy foods which when they are left in salt they secrete and remove their bitterness and become hard[17], as well as beans and lentils[18] which were cooked with their peel of which salt softens their hardness which they have as a result of their peel[19] [See Q&A], as well as species of raw cucumbers (called ugerkis) of which the salt helps them[20], as well as all other things which are commonly pickled[21], it is forbidden to salt more than one[22] piece at a time [even] in order to eat it right away.

The reason for this is because: as when one salts two pieces together and certainly [when he salts] more [than two pieces] it appears like one is pickling pickled foods[23] [which itself is forbidden due to cooking[24], or do due it being similar to tanning[25]]. [Regarding the reason behind the prohibition of pickling-See Halacha 3A!]
Not to salt for a later use:[26] [Furthermore] even one piece of radish and the like is forbidden to dip into salt and leave it [with the salt] for a long time in order so its sharpness dissipate through it secreting moisture, being that doing so is similar to tanning.

Dipping many pieces individually to eat right away:[27] However it is permitted to dip even a few pieces [when dipping] each one individually and one places it in front of him in order to them eat right away, one after the other without much delay. Even if the [pieces] remain a small amount of time [in their salt prior to being eaten] and they [thus] secrete some moisture [nevertheless] this poses no problem so long as that they do not remain a long time [with their salt] even [when planning to eat them] within the same meal [as nevertheless it does not appear like pickling[28]]. [Meaning] for example [if] from the beginning of the meal to the end [of the meal there is enough time] for [the salted food] to secrete a lot of moisture [then it is forbidden to delay eating it until the end of the meal] because doing so is similar to tanning which is forbidden [to do] with even only one piece [of food].

Other Opinions: 

[However] there are opinions[29] which say that it is forbidden to leave [the food] in salt at all even for a small amount of time, [and] even [when it is] only a single piece. Therefore [according to this view] it is forbidden to dip two pieces [in salt even] one after the other and then place them in front of him to eat immediately one after the other being that until one [finishes] eating the first piece the second piece remains in the salt. [Thus] it is only permitted to dip a single piece and then eat it right away.

The reason behind why one may not salt more than one piece at a time according to the 2nd opinion:[30] According to the latter opinion the reason that [the Sages] prohibited to salt a few pieces at the same time is not because it appears like [one is] pickling pickled [foods], but rather is because that until one [finishes] eating the first [piece] the second piece remained in the salt and since the salt benefits it, this is similar to tanning. [See Q&A]

Salting many pieces at the same time and then immediately adding vinegar or oil:[31] Therefore [based on the latter opinion] that which [people are] accustomed [on Shabbos] to cut a radish very thin[32] and then place it on a plate and salt it and then pour on it vinegar involves no prohibition even though that this is similar to salting many pieces together which [we said above] is forbidden [to be done] even to eat right away, [as] nevertheless since [the radish pieces] do not remain at all alone with the salt, as one immediately pours on it vinegar and other species, it is [therefore] not similar to tanning. Certainly [this allowance applies] if one pours oil on it being that oil weakens the strength of the salt.[33] [See Q&A regarding how close to the meal this must be done]

Nevertheless one must beware to pour the oil or vinegar [on the salted pieces] immediately after salting it [and doing so may not be delayed].[34] However that which some [people] are accustomed to make a vegetable (called lettuce) in which they first salt the vegetable on its own and then leave it this way and drain the water that comes out from it and then [only] afterwards mix it with oil and vinegar, doing so is a complete prohibition and is more similar to tanning [than when it is not left at all alone its salt] being that he delays until [the food] absorbs the salt well.[35] [See Q&A]

The Final Ruling:[36] 

The custom is like this latter opinion [that one may not delay the eating of the salted food at all] and so is the law with regards to any [food] which salt benefits. [See Q&A]