In a frying pan over medium-high heat, melt the butter. Add the onion and garlic. Sauté until fragrant, about 2-3 minutes.
Add the sliced zucchini to the skillet. Sauté, stirring frequently, until the zucchini is slightly browned, about 5-8 minutes. Be sure to keep turning and moving them around so they don’t burn.
Remove from heat. Add salt and pepper to taste and sprinkle on the Parmesan cheese. Serve immediately.