Begin by preheating your grill or grill pan to medium-high heat, aiming for a temperature of 375°F to 450°F.

Rinse and slice the zucchini, eggplant, and red bell pepper into uniform ¼-inch thick pieces.

In a large mixing bowl, toss the sliced vegetables with olive oil, balsamic vinegar, salt, and pepper until well coated.

Carefully place the marinated vegetables on the preheated grill and grill for about 4-5 minutes on each side.

Spread a layer of pesto on one side of each slice of rustic sourdough bread, then layer the grilled vegetables, fresh spinach, and mozzarella on top.

Brush the outer sides of each sandwich with olive oil, place on the grill, and grill for about 3-4 minutes per side.

Let the sandwiches rest for a minute before slicing. Garnish with fresh basil leaves and serve immediately.