Wash squash under cold running water. With a sharp knife, slice zucchini and/or summer squash into 1/2-inch thick slices. You should have about 2-2/3 cups.
To slice the onion, cut a small slice off one end of the onion. Peel the outer, paperlike skin from the onion. Starting at the cut end, cut the onion into 1/4-inch-thick slices and separate each slice into rings. With a garlic press or a sharp knife, mince the garlic clove (mincing means to chop into tiny pieces).
In an 8-inch skillet heat the oil over medium-high heat for 1 to 2 minutes or just until hot. Add zucchini and/or summer squash, onion, garlic, and Italian seasoning. Cook, uncovered, over medium-high heat for 5 minutes, stirring occasionally.
With a large spoon, carefully add undrained stewed tomatoes to the skillet. Cover skillet. Cook over medium-low heat about 3 minutes or until zucchini and/or summer squash is crisp tender. Spoon vegetable mixture onto 4 dinner plates.
Sprinkle some of the mozzarella and Parmesan cheeses on top of the vegetable mixture on each plate. Makes 4 side-dish servings.