Question:
I used a dairy cutting board to cut an onion with a meat knife. What is the status of the onion, the cutting board, and the knife?

Answer:
While this matter is subject to some debate among the poskim, the practical ruling is that everything remains kosher. All that is required is to wash the knife and cutting board thoroughly with soap and water.

Sources:

See Chochmas Adam 56:2; Tuv Taam Vadaas 3:215; Daas Torah 96; Sefer Yehoshua Pesakim Ukesavim 122, brought in Daas Torah 96 [p. 260]; See Sefer Hakashrus 10:110; Sefer Davar Charif 1:13 footnote 57; See our corresponding Sefer The Practical Laws of Meat & Milk Chapter 5 Halacha 3!