Heat about 2 tbsp of olive oil in a pan and then add the bell pepper and onion on high heat. These take the longest to cook.
After about five minutes, add the zucchini. Move the pan to low heat and let the vegetables cook.
In a separate pan heat 3 tbsp of olive oil and add the eggplant. The eggplant needs to cook separately in a significant amount of oil because it is very dry and needs to absorb the oil so that it will have the same texture as the other vegetables.
When the eggplant is cooked (it should be very tender), add it to the other vegetables.
Add the diced tomatoes to the vegetables, and cover the pan with a lid. Let the vegetables simmer on low heat for at least 20 minutes (you can leave them simmering for much longer if necessary).
Salt and pepper to taste!