Take 2 tablespoons of butter, onion, and garlic and bring to a slight simmer on low heat. Add 1/2 cup of white wine and bring to a low boil. Add flour, mix well with whisk while on low heat. (will form a rue).
While rue is forming, boil crab legs, cool, and then shell, place crab meat aside.
After rue forms, mix in heavy whipping cream, mushrooms, pepper, and parmesean cheese. Let simmer for 20 minutes.
Add crab meat, and let sauce thicken for 5 minutes.