Clean the mushrooms under cool water, pat them dry with a paper towel, and allow them to sit on a paper towel to dry. Note: Never let mushrooms soak in water, as they will lose flavor and absorb so much moisture they will be hard to sauté.
Preheat a skillet with a little bit of hot oil over medium heat. Add the diced onions and cook until softened.
Add the whole mushrooms to the skillet and season them with a bit of salt. Cook the mushrooms until they soften. Press fresh garlic into the skillet and stir to combine.
Stir the grated parmesan into the skillet. Serve immediately and enjoy!