Put the onions in a baking pan and rub them with a little olive oil.
Prick the eggplants in several places with a fork and place them in a baking pan.
Char the peppers over an open flame or under the broiler until blackened on all sides. Transfer to a plastic container with a lid or a paper or plastic bag. Cover the container or close the bag and let the peppers steam for about 15 minutes. Peel the skins from the peppers; then cut in half, remove the stems, and scrape out the seeds. Cut the flesh into strips.
Combine the onions, eggplants, peppers, and tomatoes in a large mixing bowl.
Whisk the olive oil, lemon juice, and garlic together, then season to taste with salt, pepper, and optional cumin. Pour over the eggplant mixture and gently toss to coat. Taste and adjust the seasoning and sprinkle with the parsley.