Butter a 9×13″ baking dish.
Cook onion with sausage in skillet, crumbling as it cooks.
Beat eggs, heavy cream, milk, sour cream and seasonings. Add cooked sausage and onions to mixture. Pour into prepared baking dish.
Top with green chilies and some cheese.
Top with tater tots and remaining cheese.
Cover and refrigerate overnight. (This can be made just before baking, but the tater tots should be thawed.)