Heat a large pan over medium heat and add the olive oil. Let it warm until it shimmers.

Add the sliced onion and minced garlic. Sauté, stirring, until the onion is translucent and fragrant, about 3 to 4 minutes.

Add the zucchini, bell pepper, and cubed eggplant. Stir so the pieces are evenly coated in oil. Cook, stirring occasionally, until the vegetables begin to soften, about 5 to 7 minutes.

Stir in the diced tomatoes, a pinch of salt, a few grinds of black pepper, and oregano to taste. Mix well so the tomatoes release their juices.

Lower the heat slightly and simmer for another 5 to 10 minutes, until all the vegetables are tender but not mushy. Taste and adjust salt and pepper.

Serve hot and scatter crumbled feta over the top if desired.