In a large non-stick (I like ceramic) skillet over medium heat, cook sausage, onions, mushrooms, and garlic until sausage is no longer pink and mushrooms have given off some of their liquid, about 10 minutes. Throughout cooking, stir to crumble sausage and break up mushrooms. (You can leave the mushrooms in larger slices or break them into almost unnoticeable little pieces as you crumble the sausage.) Drain the liquid.
In a 9x13 oiled pan, layer potatoes (there is no need to thaw them), sausage mixture, 1 cup cheese, green peppers, tomatoes, and green onions.
In a medium bowl, combine eggs, milk, parsley, salt, basil, and pepper. Whisk to combine, then add remaining 1 cup of cheese and whisk again. Pour egg mixture evenly over other ingredients in baking pan.
Preheat the oven to 375°F.
Bake casserole, uncovered, for about 60-70 minutes, or until egg in middle is just set and edges are lightly golden brown.
Remove from oven let stand 10 minutes.
Make ahead, cover with plastic wrap and refrigerate overnight.