Grease a 9x13-inch casserole dish or an equivalent sized pan.
Heat a large skillet over medium heat; cook the onion in the oil until it starts to get soft and translucent. Add the peppers and fry for a few minutes, then add the sausage and and stir fry until the sausage is crumbly and completely browned, and the peppers are tender, about 10 minutes. Salt and pepper the mixture. Transfer cooked sausage to the prepared casserole dish.
Mix the eggs, milk, Italian seasoning,mustard powder, salt, pepper and cheeses together in a large bowl; mix completely. Add in the bread cubes and coat completely. Pour over sausage. You can leave it as a layer OR mix it together like I did. I prefer it mixed together! Cover the casserole dish and refrigerate, overnight.
Preheat oven to 350 °F.
Bake in the preheated oven until a knife inserted into the center of the strata comes out clean, around 65-75 minutes.
Feel free to play around with the types of vegetables you want to use in this!