In a large, heavy pot, heat the oil over moderate heat, then add the onions, celery, bell pepper, and garlic, and cook, stirring, till softened, about 2 minutes.
Add the squash and cook, stirring, 3 minutes longer to soften.
Add the remaining ingredients and stir well.
Reduce the heat to a low simmer, cover, and cook for 1 hour, adding a little water if the stew begins to get too thick.