1. Rinse eggplant. Trim off and discard stems. Cut eggplant into 2-inch cubes.
2. In a 9- by 13-inch pan, mix eggplant, onions, and oil.
3. Bake in a 450° oven, stirring occasionally, until eggplant is very soft when pressed, about 45 minutes.
4. Drain and rinse garbanzos. Drain tomatoes and reserve juice. Measure juice and add water to make 1 1/3 cups.
5. Add garbanzos, tomatoes, and juice mixture to eggplant.
6. Continue to bake, stirring occasionally, until vegetables are hot, about 20 minutes. Add salt and pepper to taste.