4 tablespoons olive oil, divided, plus additional for drizzling
2 onions, diced
2 tablespoons tomato paste
3 garlic cloves, minced
1 pound eggplant, cut into 1-inch cubes
1 pound zucchini, cut into 1-inch cubes
2 large bell peppers (about 1 pound), any color or a combination, cut into 1-inch pieces
2 cups diced fresh tomatoes or one 14 ½-ounce can diced tomatoes
½ teaspoon kosher salt
½ teaspoon pepper
2 tablespoons fresh oregano or 1 tablespoon thyme leaves
In a large skillet over medium heat, heat the oil. Add the onions and cook, stirring frequently, until softened and lightly browned, about 15 minutes.
Stir in the tomato paste and garlic until well-combined. Transfer the mixture to the slow cooker.
Add the eggplant, zucchini, bell peppers, tomatoes, salt and pepper.
Cover the slow cooker, and cook on high for 4 hours on high or on low for 8 hours. To thicken the sauce a bit, set the lid of the slow cooker ajar for the last 30 minutes of cooking.
Just before serving, stir in the oregano or thyme.
Serve hot.