Favorite Grilled Cheese Sandwich

Ingredients (5)

• 2 large slices of whole grain sourdough, or your favorite crusty bread
• Enough Dijon mustard for a thin layer across one slice of bread (about 1 1/2 teaspoons)
• 1 cup lightly packed (3 ounces) freshly grated sharp cheddar cheese, plus a little more for sprinkling on the outside (optional)
• 2 teaspoons chopped green onion, green parts only
• Unsalted butter, for melting in the pan (about 2 teaspoons)