French Onion Strata Bake

One of my favorite ways to use up old bread is to make a bread pudding out of it, and this savory caramelized onion one is my current go-to. I wanted to highlight the flavors of the French onion soup with this, but you are welcome to add other veggies (spinach, roasted sweet potato pieces, bell pepper slices, and so on) and herbs (put hearty ones like rosemary into the bake and save softer herbs like parsley or dill for topping) you may have on hand along with leftover meat such as the