Cut into thin slices, discarding stems and bottoms.
Slice onion into thin rings.
Spray a wok or large, covered frying pan with cooking spray.
Melt butter in pan over medium-high heat.
Add zucchini, squash and onion.
Stir-fry for several minutes, until tender-crisp.
Season with salt and pepper, to taste.
Add 3 T water and cover.
Reduce heat and simmer for 6-8 minutes or until mixture meets your criteria for doneness (We like ours slightly soft, others may prefer either crisp or very soft).