Heat oil in a large heavy-bottomed skillet, such as cast iron.
Add the onion to the skillet and cook, stirring occasionally, for 5 minutes to soften.
Add the zucchini and squash to the skillet and cook, stirring occasionally, for 5 minutes.
Add the tomato, garlic, salt, basil, and pepper, and stir well. Continue cooking, stirring occasionally, for 5 minutes or until the squash has softened to your liking.
Sprinkle with fresh parsley before serving.
Video
Notes
You may use fresh basil in place of dried, if you prefer. Add to taste.