Preheat oven to 400°F.
Wash, dry and trim the zucchinis. Cut each one in half (lengthwise) and, using a spoon, scrape out the zucchini seeds, making four long canoe-like boats.
Brush the zucchini with one tablespoon of olive oil. Season with crushed garlic.
Arrange zucchini boats on a baking sheet, and bake uncovered for 20 minutes
Toss together halved grape tomatoes, finely diced red onion, remaining two tablespoons of olive oil, balsamic vinegar, and basil. Season generously with salt and pepper to taste.
Remove zucchini boats from the oven and arrange on four serving plates. Top with tomato mixture and serve.