Heat the ghee over medium-high heat in a large skillet.
Add the sliced onions and cook for about 5 minutes, stirring frequently until softened but not brown.
Add the sliced mushrooms and garlic. Season with salt and pepper and sauté. The key here is not to stir the mushrooms too much. This will allow them to brown and caramelize giving you a rich, umami flavor. Cook until the mushrooms and onions are lightly brown and tender about 8 minutes.
Add the spinach. Stir just until the spinach wilts, scraping up all the little bits of flavor from the bottom of the pan with the liquid from the spinach.
Taste to adjust salt and pepper. Serve.
Notes
Shiitake, hen of the woods or chanterelle mushrooms can also be used for a more exotic dish.