1. Heat butter in large nonstick skillet over medium-high heat. Add mushrooms; sauté 5-6 minutes.
2. 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.
3. 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.