In a large skillet, heat 1 tablespoon of olive oil and 2 tablespoons of butter over medium-high heat until the butter melts and foams.

Add the chopped medium yellow onion and white button mushrooms to the skillet. Sauté for about 5 minutes until the onions are translucent and the mushrooms are browned.

Once the onions and mushrooms are nicely caramelized, carefully plate them and set aside. Add a splash of olive oil if the skillet appears dry.

Add the chopped medium zucchini to the skillet, spreading it in a single layer. Cook undisturbed for about 3 minutes before tossing gently. Sauté for another 5-7 minutes until browned and tender.

Reduce the heat to low, stir in 2 minced garlic cloves, 1 teaspoon of Italian seasoning, and a pinch of red pepper flakes. Cook for an additional minute until garlic becomes fragrant.

Taste the mixture and season with kosher salt and freshly ground black pepper according to preference. Stir everything together and let it sit for 30 seconds to meld the flavors.

Transfer the sautéed zucchini to a serving dish and enjoy promptly.