Wash the zucchini and yellow squash thoroughly. Slice the squash into ¼ inch rounds or half moons. Cut the onion into thin slices.
In a large 12-inch skillet, heat the butter and the olive oil over medium heat. Add the zucchini, yellow squash, and the onion to the skillet. Toss the veggies with the butter and oil then season with half of the salt and black pepper.
Now let the squash cook without stirring for 2 minutes on one side so the squash can get a golden brown color. Then gently stir the squash and cook on the opposite side for 3-4 minutes, stirring occasionally, until the squash is tender to your liking. Season with the rest of the salt and black pepper. Enjoy!