Put the onion, bell peppers, eggplant, zucchini, grape tomatoes, garlic cloves, thyme sprigs and rosemary sprigs on the prepared baking sheet, and season with salt and pepper. Drizzle with ¼ cup of the olive oil and 1 tablespoon of the balsamic vinegar. Toss well to coat. Spread the veggies in an even layer (it’s okay if the vegetables are touching and overlapping). Roast until the vegetables are tender and the tomatoes have popped, stirring twice during cooking, about 35-40 minutes.