Beef and Sweet Onion Sandwiches

12 ounces boneless beef sirloin or top round steak, cut 1-inch thick
1/2 teaspoon coarsely ground black pepper
Salt
2 teaspoons vegetable oil
1 medium sweet onion, sliced
2 tablespoons Dijon-style mustard
1/2 cup roasted red bell peppers (1/2 of a 7-ounce jar, drained)
8 (1-inch-thick) slices sourdough
1 1/2 cups torn spinach or salad greens

Trim fat from meat. Sprinkle both sides of meat with pepper and salt; press in lightly. In a large skillet cook meat in hot oil over medium-high heat about 8 minutes or until slightly pink in center, turning once. Remove from skillet; keep warm. Add onion to drippings in skillet, adding more oil if necessary. Cook and stir about 5 minutes or until onion is crisp-tender. Stir in mustard; remove from heat. Meanwhile, cut roasted red peppers into 1/2-inch wide strips. Toast bread, if desired, and shred spinach or salad greens. Just before serving, thinly slice meat into bite-size strips. To serve, top 4 bread slices with spinach or salad greens, meat strips, roasted pepper strips, onion mixture, and remaining bread slices.
Makes 4 servings.