Brown the breakfast sausage in a large skillet over medium heat, breaking it into small pieces, then drain extra fat so the casserole stays firm.
Grease a baking dish and spread half of the frozen tater tots in an even layer on the bottom to make a crisp potato base.
Scatter the cooked sausage over the potatoes, then sprinkle the diced onion across the top so every bite has gentle sweetness and crunch.
Cover the sausage and onion with shredded cheese, then add the remaining frozen tater tots over the surface to form a golden topping.
Whisk the eggs with milk, salt, pepper, and herbs in a bowl until the mixture looks smooth and the seasoning is spread through.
Pour the egg mixture slowly over the layers in the baking dish and bake at 375 F for about 40 to 45 minutes until the center feels set and the top looks brown and bubbly.