Fry the champignons. To do this, pour oil into a frying pan and heat it up. Put the champignons in and fry at maximum heat. Stir the mushrooms occasionally. Fry until evenly golden, about 10-15 minutes.
Finish cooking. Add chopped onion to the mushrooms. Reduce heat to medium and cook until mushrooms and onion are fully cooked, about 15 minutes. Stir, season with salt and pepper, and remove from heat.