Preheat oven to 350 degrees F. Lightly oil an 8x11, 8x8 or similarly sized baking dish.
Optional: Cook the hash browns in a skillet for 5-6 minutes, until lightly golden. They don't need to be cooked all the way through but should be thawed.
Cook veggie sausages according to package instructions. Use a knife or spatula to crumble up into bite-sized pieces.
Heat a large skillet over medium-high heat. Once hot, drizzle in olive oil. Add the bell peppers and mushrooms, sautéing until softened, about 5-6 minutes.
In a large mixing bowl, use a whisk to mix together the eggs, milk, salt, garlic powder, onion powder and black pepper as if you were making scrambled eggs. Set aside.
Assemble the casserole: Layer the hash browns in the lightly oiled casserole dish. Add the crumbled sausage and sautéed peppers and mushrooms. Sprinkle on 1 cup of cheddar cheese. Pour the eggs over the cheese, making sure to cover all dry areas.
Bake uncovered for 35-40 minutes, or until golden brown and the eggs are cooked through at the center. The edges should be turning golden and pulling away from the pan.
Let rest 5 minutes before slicing and serving. Enjoy!
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Notes
Hash browns need to be thawed before adding to the casserole. I usually cook them in a cast iron skillet for a few minutes to lightly brown them and add flavor, but this is optional.
Veggie sausage: I use Morningstar maple-flavored breakfast sausages or Lightlife links. You can also replace vegetarian sausage with equal amounts of sautéed mushrooms.
To make ahead: Vegetarian egg casserole can be prepared up to 24 hours in advance. To make ahead, assemble the casserole according to the recipe instructions. Cover with foil or other lid and refrigerate (I love these baking dishes because they come with reusable lids). When ready to make, remove the lid and bake in the oven for 35-40 minutes at 350F.
Leftovers and reheating: Egg casserole will last in the fridge for 3-5 days and can be reheated in the microwave in about a minute.
Freezing: Freeze individual portions wrapped in foil or freeze-safe containers for up to 3 months. Reheat individual pieces in the microwave in 1-2 minutes.