Cooking & Baking

I did kosher catering for a summer in college so I'm actually not entirely unfamiliar with it!

They were probably prepared as you said (soaking in milk does the same thing, but obvs not kosher), then sautéed in chicken fat, had some flour thrown in and cooked down into a roux, then had some white wine and chicken stock put in and simmered to thicken. The onions may well have been prepared ahead of time, proper caramelized onions only need onions and salt but take 4-5 hours.

Come to think of it, since the place I currently work at is Jewish (not Kosher but still) and we just finished prepping our Rosh Hashanah dinners…I'd bet $5 that that je ne sais quoi you're looking for is the use of chicken as the cooking fat. It's a very distinct and delicious flavor!