Preheat the grill or grill pan to medium-high heat.

In a small bowl, whisk together olive oil, balsamic vinegar, garlic powder, salt, and pepper.

Brush the Portobello mushrooms with the olive oil mixture on both sides.

Grill the mushrooms for 4–5 minutes per side, or until tender and juicy.

If using mozzarella, place a slice of cheese on each mushroom cap during the last minute of grilling to melt.

Toast the buns lightly on the grill.

Assemble the sandwiches by spreading mayonnaise and Dijon mustard (if using) on the buns.

Place the grilled mushrooms on the bottom buns, top with fresh basil leaves, and close with the top buns.

Serve immediately and enjoy!