Vegetarian Breakfast Casserole with Hash Browns

Name: Vegetarian Breakfast Casserole with Hash Browns
Ingredients: cooking spray; 4 cups frozen hash brown potatoes, thawed; salt and ground black pepper to taste; 1 tablespoon olive oil; 1 medium yellow onion, chopped; 0.25 teaspoon dried thyme; 0.25 teaspoon salt; 0.125 teaspoon ground black pepper; 4 ounces vegetarian sausage; 1 medium bell pepper, chopped; 1 cup chopped mushrooms; 4 large eggs; 2 large egg whites; 1.5 cups shredded Cheddar cheese; 0.25 teaspoon hot sauce; 1 medium tomato, sliced; 1 teaspoon chopped fresh parsley