Trim the squashes and cut them into 1/4 inch dice. In a heavy skillet, heat 2 tablespoons of the olive oil and add 1/3 of the diced squashes and some salt and pepper. Saute briefly over high heat, stirring often until lightly browned and slightly softened. Transfer to a bowl to cool and cook the remaining 2 batches in the same way, seasoning each batch. Cook the diced onion in the remaining olive oil in the same skillet over medium heat for 3 to 5 minutes. Stir in garlic and cook for 20 seconds more. Add the cumin, reduce heat to low, and cook, stirring, for about 2 minutes. Add to the squashes in the bowl. Add cooked rice, vinegar and chopped parsley, and adjust the seasonings. Serve immediately on lettuce lined plates, if desired, or refrigerate for up to 3 days.