Pre-prep: Set the potatoes out to thaw. Cook and cool the sausage.
If you are baking the casserole immediately, preheat the oven to 375°.
In a large bowl, stir the thawed potatoes and cooled melted butter together.
Press the potatoes into the bottom and sides of a greased 9x13 pan.
In another bowl, beat the eggs and ground mustard together then add the sausage, cheese, onion, and cottage cheese and stir to combine.
Pour over the potato mixture.
If you are baking immediately, place the casserole in the preheated oven and bake for 1 hour.Otherwise, cover with plastic wrap and refrigerate or freeze until time to use it. Then bake at 375° for 1 hour.
Notes:
Make Ahead: This is very easy to prepare ahead of time - assemble the casserole, cover tightly, and refrigerate or freeze until ready to bake.If frozen, let thaw in the fridge overnight before baking.*For added crunch, pan-fry the hashbrowns before layering them in the pan. You can still prepare the egg mixture the night before, but wait to pour it over the potatoes until just before baking.