Creating a grilled Portobello sandwich is a straightforward process that ensures maximum flavor and texture. Follow these step-by-step instructions to achieve the perfect sandwich:

1. Prepare the Marinade: In a small bowl, whisk together the olive oil, balsamic vinegar, minced garlic, salt, and pepper. This mixture will enhance the natural flavors of the Portobello mushrooms.
2. Marinate the Mushrooms: Clean the Portobello mushrooms by gently removing the stems and wiping the caps with a damp cloth. Place the mushrooms in a shallow dish and pour the marinade over them. Allow them to marinate for at least 30 minutes, flipping halfway through to ensure even coating.
3. Preheat the Grill: Heat your grill or grill pan to medium-high. If you’re using a grill, lightly oil the grates to prevent sticking.
4. Grill the Mushrooms: Place the marinated mushrooms on the grill. Cook for about 5 to 7 minutes on each side, or until they are tender and have nice grill marks. Near the end of the grilling time, lay a slice of cheese on each mushroom to allow it to melt slightly.
5. Prepare the Bread: While the mushrooms are grilling, lightly toast the bread slices on the grill or in a toaster to achieve a crispy texture.
6. Mix the Sauce: In a small bowl, combine the mayonnaise and Dijon mustard. Spread this mixture generously on one side of each bread slice.
7. Assemble the Sandwich: Start by placing a grilled mushroom with melted cheese on a slice of bread. Top with fresh arugula or spinach, tomato slices, and red onion. Add any optional ingredients like roasted red peppers or avocado for additional flavor and texture.
8. Finish and Serve: Top with the second slice of bread, slice the sandwich in half, and serve immediately while warm.