Add half of the onion to the pan and cook for 2 minutes. Add half of the garlic and all the bell peppers to the onion mixture in the pan. Continue to cook the vegetables in the pan and add all of the yellow squash and zucchini to the pan.
Lightly season the cooking vegetables with salt and pepper. Cook vegetables until squash is just crisp-tender (about 2 minutes).
Remove vegetables from the pan into a large serving bowl.
Cook vegetables until the eggplant is just tender (about 2 minutes). Remove the vegetables from the pan and place them into the bowl with the other vegetables.
Add the fresh herbs to the bowl of vegetables and stir to combine. Serve warm.