Melt butter in a medium-sized saucepan and fry green onions until soft. Add flour and cook, stirring constantly, for 1 minute. Remove from heat source and add milk, wine, and mustard. Return to heat source and bring to a boil, stirring constantly. Add crabmeat and cream.
Spoon mixture into six individual gratin dishes, or if you prefer, in a large baking dish.
Mash the potatoes with milk and butter and spread evenly over crab mixture. Sprinkle with grated cheese. Brown under broiler and garnish with lemon slices and parsley.