1. Heat 1 tsp. melted butter in large skillet over medium heat. Add leeks, and season with salt, if desired. Cover, reduce heat to medium-low, and cook 7 to 10 minutes, or until tender. Set aside.
2. Brush 4 slices bread with half of remaining melted butter, and set butter side down on baking sheet. Divide Provolone among bread slices, and layer each with one-quarter of leek mixture and 1 red bell pepper half (if using). Cover with remaining bread slices. Brush sandwich tops with remaining melted butter.
3. Heat large skillet or griddle over medium-low heat. Cook sandwiches in skillet 4 minutes, or until browned and crisp. Flip, and cook 3 minutes more,
or until second side is browned.