Heat a large skillet over medium-high heat and add olive oil and butter. Heat until butter is melted and bubbling, about 1-2 minutes.

Add diced onion and white mushrooms to the skillet. Sauté for about 5-7 minutes until golden and tender.

Remove the cooked onions and mushrooms from skillet. Drizzle additional olive oil if needed.

Add chopped zucchini to the skillet in a single layer. Sauté undisturbed for 2 minutes, then toss and brown for another 2-3 minutes.

Reduce heat, add garlic, Italian seasoning, and red pepper flakes. Stir and cook for 1 minute.

Season with salt and pepper to taste. Serve immediately for the best flavor.