Add 1 tablespoon of olive oil and 2 tablespoons of butter to a large skillet over medium-high heat. Allow the fats to melt and heat until they shimmer, about 1-2 minutes.

Introduce the chopped yellow onion and quartered mushrooms to the skillet. Sauté for approximately 5 minutes or until the onions become translucent and mushrooms are golden brown.

Remove the sautéed onions and mushrooms from the skillet and set them aside on a plate.

Pour a splash more olive oil into the skillet if necessary, and heat for about 30 seconds before adding the zucchini chunks.

Add the zucchini to the hot skillet, ensuring even distribution. Sauté for about 3-4 minutes until browned on the edges but tender-crisp.

Reduce heat to low and stir in the minced garlic, Italian seasoning, and red pepper flakes. Mix well over low heat for about 1-2 minutes.

Season with kosher salt and fresh ground black pepper to taste.