Heat butter and olive oil in a large skillet over medium-high heat. Add mushrooms, onions and garlic and saute over medium-high heat for about 5 minutes. Mushrooms should be nicely browned and onions browned and tender.
Add spinach, reduce heat to low, and cook about one more minute, stirring frequently until spinach is wilted.
Add the cooked quinoa to the skillet. Cook about 1 more minute to warm quinoa (if needed).
Season with salt and pepper to taste and serve hot.
Note: This is also quite delicious cold. You can also substitute olive oil for the butter if you want it to be a vegan meal.