Onion rings [i.e. breadcrumb coated rings of onion]:
Onion rings are typically made through coating a ring of an onion in a flour based dip and then deep frying them. In addition, some then coat the ring with dry bread crumbs, hence adding a further layer of Mezonos product to the ring. In both cases, one recites the blessing of Mezonos upon eating onion rings.
 Sefer Viten Bracha 4/G3 [p. 79]
Other opinions: Some write the blessing over onion rings is Hadama. [Luach Rav Prus] Seemingly, he compares this to chicken or fish Shnitzel, and other crumb coated foods, of which the custom is to say the blessing of Shehakol and not Mezonos. However, in truth, one cannot compare the two, as the coating of onion rings is thick and a main part of the food, and is similar to coated peanuts which is Mezonos. [See Piskeiy Teshuvos 208/6] Thus, his ruling regarding breadcrumbs is inaccurate, and its blessing is Mezonos.
 The reason: As all mixtures that contain a Mezonos ingredient for purposes of taste or satiation receive the blessing of Mezonos even if they are not the main ingredient. [Admur 208/1; Seder 3/2; Luach 4/2; Michaber 208/2; Brachos 36b] Now, certainly onion rings are coated for purposes of eating and tasting the bread crumbs and not just for the onion.