Heat oil in a large pan over medium heat. Add the diced onion and sauté until translucent.
Add the tomatoes to the pan, and let them simmer for a few minutes.
Incorporate the cubed eggplants, zucchini, squash, and both bell peppers to the pan.
Cover the pan and reduce the heat to low. Let the vegetables simmer for 20-30 minutes, stirring occasionally to ensure even cooking.
Season with salt and pepper to taste. Once the vegetables are tender, remove from heat.
Pro Tip: This dish can be served as a side, or if you want to add some protein, throw on top a fried egg! And of course, have a slice of bread to soak up the delicious juices!
