Creamy Mushroom and Israeli Couscous

Name: Creamy Mushroom and Israeli Couscous
Ingredients: 8 ounces cremini mushrooms or a mushroom of your choice, sliced 1/2-inch thick; 3 - 4 cups Israeli or pearl couscous (cooked already according to package); 1 small yellow onion, thinly sliced; 1 clove garlic, minced; 2 tablespoons plain Greek yogurt or sour cream; 2 tablespoons unsalted butter; 2 to 3 tablespoons olive vegetable oil (or just enough to coat your skillet); 1/4 cup water, or more if needed; Sea salt and ground pepper to taste; Fresh parsley or sage (optional); Parmesan cheese (Optional)