Spray a skillet with cooking spray and place over medium high heat. Brown the mushrooms, stirring frequently for several minutes. During cooking, cover the skillet periodically as it begins to dry out. Add a few teaspoons of water or broth to the skillet if necessary.
Add the spinach and reduce the heat to medium. Cook, stirring frequently, until the spinach is heated through and the excess liquid has evaporated.
Stir in the sunflower seeds. Remove the skillet from the heat and stir in the mayonnaise followed by the Parmesan cheese.
Place a quarter of the spinach mixture on each of 4 bread slices or on the bottom half of each bun.
Top with a tomato slice, some sprouts, and some cheese slices.
Spread the remaining bread slices or bun tops with some of the mustard and place on the sandwiches.
Cover each sandwich with a paper towel, place on a microwave safe plate, and microwave on HIGH for about 30 seconds until the cheese is melted. Serve hot.