Combine the cheeses; sprinkle half of the cheese into a 9x13x2 baking dish.

Saute the onions, peppers and mushrooms in butter until tender and then sprinkle over cheese.

Layer the potatoes and sausage in the pan.

Sprinkle with the remaining cheese.

In a bowl, combine the flour, parsley, salt, pepper and mustard. Add eggs and milk and whisk until smooth. Pour over the top.

Bake uncovered at 350°F for 60 minutes or until a knife inserted near the center comes out clean.