Heat butter in large nonstick skillet over medium-high heat.
Add mushrooms; saut 5-6 minutes.
Spread mustard on 4 slices of bread; layer each with 6 spinach leaves, 1 tablespoon pepper, 1 tablespoon onion, 1/4 cup cheese, and 1/4 cup mushrooms. Top with remaining bread.
Brush melted butter over both sides of each sandwich. If using a skillet, heat a large nonstick skillet over medium-high heat; place sandwich- es in pan.
Place another skillet on top of sandwiches; cook 2-3 minutes on each side until golden brown and cheese is melted. If using a panini maker, brush sandwiches with butter; place in machine, close, and cook 2 minutes or until done.