In a large serving bowl, toss together lettuce, arugula and basil leaves; set aside.
Place the tomatoes, zucchini, red and yellow bell peppers and onions on a large rimmed baking sheet and drizzle with olive oil. Sprinkle evenly with salt and pepper, tossing to coat all of the veggies.
Place veggies in a grill basket and grill until tender and lightly charred, about 5 minutes per side. (If using an indoor grill pan, you will need to grill in two batches to avoid overcrowding the pan and steaming the veggies.)
Remove the charred peppers onto a cutting board and chop into bite-size pieces. Add the grilled veggies to the salad. Drizzle with balsamic vinegar and season with salt and pepper.