Clean the mushrooms: Gently brush away any dirt using a damp paper towel. Cut off the tips of the bottoms. Slice any larger mushrooms in half.
Heat the butter in a large skillet over medium-high heat.
Add the onions and cook and caramelize for 4–5 minutes, stirring occasionally, until soft and golden.
Add the mushrooms to the pan. Sauté for 6–8 minutes, stirring occasionally, until the mushrooms release their liquid and turn tender and lightly browned. Note: once you add the mushrooms, they will absorb the butter quite fast. If the pan seems dry, add extra butter or oil as needed.
Season with salt to taste (and pepper if you want). Enjoy!