Spray a 7"x10" casserole with cooking spray. Set aside.
Sauté onions and mushrooms in olive oil until onions are nearly translucent, 2-3 minutes.
Add garlic, and continue to cook for another 1-2 minutes. Set aside.
In a large bowl, whisk together eggs, cream, and milk.
Stir in ham, cheddar, fontina, thyme, and onion mixture.
Add bread cubes.
Season with salt and pepper.
Pour into prepared casserole and let stand for 15-20 minutes, until liquid is absorbed.
Bake for 35 minutes, or until set.
Notes
Substitutions:
You can use gruyère, provolone, or gouda instead of fontina.
We used hickory smoked ham, but feel free to use your favorite kind!
Expert Tips:
Make sure you let the croissants soak up the custard! It's the best way to get all the flavor.
When the casserole is "set" should still have a little bit of jiggle/movement in the middle. If you bake until fully set (no jiggle/movement in the middle), the casserole will be overcooked because it still cooks even after you get it out of the oven.