In a small skillet melt the butter and add sliced mushrooms, brown over medium heat to get some colour on them. (If mushrooms release too much liquid and are steaming instead of browning, turn up the heat.) Add a pinch of salt and pepper once they are nearly browned.
Once mushrooms are browned turned the heat to low and add finely chopped onions, (add 1 tbsp of butter if the skillet is too dry) and cook the onions for 10 minutes until they are limp and translucent, stir once in a while to prevent burning. (Don't rush this step, it's important that the onions are fully cooked.) Then add minced garlic and stir fry for 30 seconds.
Pour in cognac or brandy and let it bubble for 1-2 minutes, then add cream, creme fraiche if using, salt, pepper and chopped parsley, stir and let it simmer for a few minutes. Taste and adjust seasoning if needed. Serve with steak, chicken, pork chops or roasted vegetables.