Preheat grill to medium-high heat (400°F/200°C) for 10 minutes. Clean grates with a grill brush and oil lightly with a paper towel dipped in olive oil to prevent sticking.
Slice zucchini lengthwise into ¼-inch thick planks, bell peppers into quarters removing seeds, and red onion into ½-inch thick rounds. Brush all sides with 1 tablespoon olive oil and season with salt and black pepper.
2 medium zucchini, 2 units bell peppers, 1 large red onion, 1 tsp salt, 0.5 tsp black pepper, 3 tbsp olive oil
Place vegetables in a single layer on the grill. Grill for 3-4 minutes per side without moving them, until visible grill marks and slight char appear. Flip gently with tongs, keeping 1-2 inches between pieces to avoid steaming.
2 medium zucchini, 2 units bell peppers, 1 large red onion
Remove vegetables when tender yet firm with charred edges. Transfer to a cutting board and let rest for 5 minutes to cool slightly, then chop larger pieces into bite-sized chunks while still warm.
2 medium zucchini, 2 units bell peppers, 1 large red onion
In a large bowl, toss the vegetables with 2 tablespoons balsamic vinegar and torn fresh basil leaves. Adjust salt to taste and serve immediately or at room temperature within 2 hours.
2 medium zucchini, 2 units bell peppers, 1 large red onion, 2 tbsp balsamic vinegar, 0.25 cup fresh basil, 1 tsp salt
