Slice the zucchini and yellow squash into 1/2-inch thick rounds, cut the red bell pepper into 1-inch wide strips, and slice the red onion into thick pieces (keeping them intact so they don't fall through the grill grates). Place all the prepared vegetables and grape tomatoes on a large sheet pan or baking sheet. Drizzle with the 3 tablespoons of olive oil and season evenly with 3/4 teaspoon salt and 1/2 teaspoon black pepper, tossing gently to coat all pieces.

Preheat your grill to medium-high heat or position an oven rack 4-6 inches from the broiler element and preheat the broiler. Grill the vegetables for 10-15 minutes, turning halfway through cooking, until they develop nice char marks and are tender. If using a broiler, arrange vegetables on a baking sheet and broil for 10-15 minutes, stirring once or twice. I like to get a good char on the vegetables because it caramelizes the natural sugars and adds depth to the salad—don't be afraid of those dark spots. Transfer the grilled vegetables to a plate and let them cool to room temperature.

While the vegetables are cooling, bring a large pot of salted water to a boil and cook the rotini pasta according to package directions, typically 7-10 minutes for al dente texture. Drain the pasta and rinse briefly with cool water to stop the cooking process, then set aside to cool. Meanwhile, whisk together the 1/3 cup extra virgin olive oil, balsamic vinegar, mayonnaise, Dijon mustard, honey, pressed garlic, dried basil, 1/2 teaspoon salt, 1/4 teaspoon black pepper, and red pepper flakes in a small bowl or jar until emulsified and well combined. This dressing can be made ahead, so I like to prepare it while the pasta cooks to maximize efficiency.

Once the grilled vegetables have cooled enough to handle, chop them into bite-sized pieces (roughly 1/2 to 3/4-inch chunks so they're easy to eat with pasta). Roughly chop the fresh Italian parsley. In a large serving bowl, combine the cooled pasta, chopped grilled vegetables, and parsley. Pour the dressing over the salad and toss everything together until well combined and evenly coated. Let the salad sit for 5-10 minutes before serving to allow the flavors to meld together—I find this resting time really helps the vegetables flavor the pasta.