Couldn't be easier. You may even have the ingredients in the house *right now*.
Bread (french, old/er, cubed), milk (2 cups, whatever type you have), eggs (2), cheese (1 cup, I used gruyere, but any would be good), thyme, salt, pepper, and onions (caramelized, I used 1 yellow, 2 red).
Beat wet ingredients, reserve 1/4. Add bread + 1/2 onions + 1/2 cheese to mixture. Pour in greased casserole dish, top with remaining cheese, onions and milk mix. 425 degrees until golden brown, approx 30 minutes.