Preheat the grill to medium-high for direct-heat cooking. In a bowl, whisk together the oil, vinegar, minced garlic, salt, and pepper.

Lightly coat the zucchini and eggplant slices with cooking spray. Grill the vegetables directly on the grill grate for 3-4 minutes per side, until just tender, flipping once.

Transfer the grilled eggplant and zucchini to separate bowls. Toss each with 2 tablespoons of the prepared dressing.

Using a long serrated knife, slice off the top third of the loaf of bread. Hollow out the bottom portion, leaving a ½-inch border. Reserve the removed bread for another use.

Spread pesto evenly on the cut sides of both the top and bottom portions of the bread.

Slice the roasted red peppers into thin strips. Layer the cheese in the bottom portion of the bread.

Add the eggplant slices on top of the cheese. Layer half of the zucchini slices over the eggplant.

Place the red pepper strips on top of the zucchini. Drizzle with the remaining dressing.

Add the remaining zucchini slices, followed by the spinach. Place the top of the bread back on.

Wrap the entire loaf tightly in plastic wrap or aluminum foil. Refrigerate for at least 2 hours or overnight. When ready to serve, cut the sandwich into 8 wedges.