Spread 8 slices of bread with 1 tablespoon of mustard.
Place one 1-ounce slice of Swiss cheese on eight of the slices of bread.
Divide the sausage into eight portions and distribute evenly on the eight cheese-topped bread slices.
Top the sausages with the remaining Swiss cheese.
Close the sandwiches with the remaining eight slices of bread.
Brush the outsides of each sandwich with melted butter or spray with cooking spray.
Cook in a panini or other sandwich press; OR cook in a nonstick saute pan set on medium heat (weight each sandwich with a cake pan holding a 28-ounce can; cook on one side until golden brown, remove the weight, turn the sandwich, and cook on the second side until golden brown and the cheese melts).
Slice each sandwich in half and serve with soup or chips.