Ad the duck fat and mushrooms to a large pan set over medium heat. As the fat melts, coat each of the mushroom slices. Once all the mushrooms are coated, add the garlic, and salt, and let the contents of the pan gently brown. Once one side of mushrooms brown, gently stir the contents to brown the second side.
Once the mushrooms are nicely browned, add in the 3 cups of spinach and cover the pan to gently steam. Once the the spinach leaves have just barely begun to wilt mix the contents of the pan and serve.
This dish is best served hot, and does not keep well.