Cut the onion into thin half rings. Cut the leek into thin rings. Chop the rosemary. Cut the champignons into medium-thick slices. Chop the garlic.
Melt half of the prepared butter in a frying pan with high sides.
Fry the onion until golden brown and fragrant for 3-5 minutes. Place the sliced champignons in the pan. Add the remaining butter and garlic to the pan. Simmer the mushrooms and onion for 5-7 minutes until the champignons are done.
Add cinnamon, salt and pepper to taste. Mix the ingredients thoroughly.
Carefully pour the cream into the pan. Mix the ingredients thoroughly and add chopped rosemary. Simmer the mushrooms in the cream for another 2-3 minutes.
Place the champignons in cream on serving plates, garnish with chopped leeks. Add salt and pepper to taste. Serve the dish hot. Enjoy!