Heat the oil in a large skillet over medium-high heat and then add sliced mushrooms. Sprinkle with salt and pepper. Saute for 3-4 minutes until the mushrooms start to soften.
Sprinkle the sugar over the mushrooms. Add the garlic and cook for about 1 minute. Add the spinach and cook until wilted. Note: You may need to add the spinach in batches depending on the size of your pan.