Preheat oven to 425 degrees. Tear croissants into large bite sized pieces and spread on an even layer on a baking sheet.
Toast for 3-5 minutes in the oven. Keep a close eye as they can brown quickly. Remove when golden brown and allow to cool.
Meanwhile, heat a large skillet over medium high heat. Brown the breakfast sausage then remove from pan and set aside.
Add olive oil to pan, then add onions and bell peppers, along with a pinch of salt. Sauté until soft and golden, 5-10 minutes. Set aside to cool.
Prepare a 9x13 baking dish by liberally buttering the inside, set aside.
In a large bowl, whisk together eggs, milk and half and half, salt, pepper, garlic powder.
Add browned sausage, peppers, onions, and 1 and 1/2 cups of grated gruyere cheese.
Add croissants and toss until everything is fully combined and croissants are fully moistened.
Transfer contents of the bowl to the baking dish, pouring any excess liquid on top of the mixture. Arrange mixture in an even layer. Top with a piece of parchment paper (keeps foil from sticking), than cover with foil. Refrigerate at least 1 hour or overnight.
When ready to bake, remove from fridge about 30 minutes before baking. Sprinkle remaining half cup of cheese over the top evenly. Preheat oven to 350 degrees F.
Bake covered for about 30 minutes. Remove cover and bake for 10-15 minutes longer, until top is golden and there is no visible liquid. Allow to sit for 15 minutes before slicing and serving.
Notes
Leftovers are good for 2-3 days in a sealed container in the refrigerator.To freeze leftovers, wrap in foil and then place in a freezer bag. Leftovers can be frozen for up to 4 weeks.To reheat, wrap in foil and warm in a 350 degree oven for 5-10 minutes, or until heated through.