Grease a 9x13-inch baking dish and preheat your oven to 375°F.

In a large skillet over medium-high heat, cook the breakfast sausage, breaking it up with a wooden spoon until browned and cooked through, about 5-7 minutes. Transfer to a paper towel-lined plate to drain.

In the same skillet, add olive oil if needed. Add onion, bell peppers, and mushrooms. Sauté until vegetables are softened, about 5-6 minutes. Add garlic and cook for another minute until fragrant.

In a large bowl, combine the thawed hash browns, cooked sausage, sautéed vegetables, and 1 1/2 cups of shredded cheddar cheese. Toss to mix well. Transfer this mixture to the prepared baking dish, spreading it evenly.

If using diced ham, sprinkle it evenly over the hash brown mixture.

In another bowl, whisk together eggs, milk, heavy cream, dried herbs, salt, black pepper, and cayenne pepper (if using) until well combined. Stir in green onions and parsley.

Pour the egg mixture evenly over the hash brown mixture in the baking dish.

Cover with aluminum foil and bake for 40 minutes.

Remove foil, sprinkle the remaining 1/2 cup of cheddar cheese on top, and return to the oven uncovered for an additional 15-20 minutes, or until the center is set and the top is golden brown.

Let the casserole rest for 10 minutes before serving. Garnish with additional fresh parsley.