Wash and dry all vegetables. Slice the zucchini into ½-inch lengthwise pieces, cut the red pepper into strips, cut the red onion into thin rings, and slice the portobello mushrooms. Put everything into a large bowl.

In a small bowl, whisk 2 tbsp olive oil, the balsamic vinegar, honey or maple syrup, minced garlic, sea salt, and black pepper until combined.

Pour half the glaze over the vegetables. Toss until lightly coated. Let them rest for 5 minutes while the grill heats.

Preheat the grill to medium-high (around 400°F). Lightly oil the grates to prevent sticking.

Grill the marinated vegetables for 3–4 minutes per side until tender-crisp with nice char marks. Remove to a plate.

Brush the cut sides of the ciabatta with the remaining glaze and grill cut-side down for 1–2 minutes until lightly crisp.

While the bread is still on the grill, place mozzarella slices on the bottom halves, covering with a lid for 30–45 seconds to melt.

Pile the grilled vegetables on top of the melted mozzarella, drizzle with any remaining glaze, and scatter torn basil leaves.

Close the sandwich, press gently, and give it a final 1-minute press on the grill to warm through.

Remove, let rest for a minute, then slice and serve warm.