Preheat your oven to 375°F (190°C). This ensures your casserole cooks evenly and comes out perfectly golden.
In a skillet over medium heat, add a splash of oil. Once hot, add the diced onion and bell peppers. Sauté until softened, about 5-7 minutes. Then, add the crumbled vegan sausage and cook until heated through and slightly browned. Stir in the Italian seasoning, garlic powder, onion powder, and paprika. Remove from heat and set aside.
In a large mixing bowl, combine the vegan egg substitute, flour, baking powder, and a pinch of salt. Whisk until smooth. Then, fold in the sautéed vegetable and sausage mixture, followed by the frozen hash browns and shredded vegan cheddar cheese.
Spread the mixture evenly into a greased 9x13-inch baking dish. Use a spatula to flatten the top, ensuring an even bake.
Place the casserole in the preheated oven and bake for 40-45 minutes, or until the top is golden brown and the edges are crisp. A toothpick inserted in the center should come out clean.
Once out of the oven, allow the casserole to cool for a few minutes. Garnish with freshly chopped parsley or green onions before serving.