Balsamic Roasted Broccoli and Red Pepper Grilled Cheese

Ingredients

• 1 pound broccoli, or 8 ounces broccoli florets
• 1 red bell pepper
• 1 small yellow onion
• 2 tablespoons olive oil
• 2 tablespoons balsamic vinegar
• ¼ teaspoon salt
• Pinch of red pepper flakes (optional, for spice)
• Freshly ground black pepper, to taste
• 8 slices quality whole grain bread, preferably sourdough
• 12 ounces shredded cheddar cheese
• 1 to 2 teaspoons Dijon mustard
• 1+ tablespoons butter