Melt butter in a large saucepan over medium low heat. Whisk in flour and cook, stirring, for 3 minutes. Gradually whisk in milk. Add onion, stock powder, parsley, salt, pepper, and nutmeg. Increase heat and bring to a boil, then reduce to a simmer and cook until the soup begins to thicken, stirring occasionally--about 10 minutes. Add crabmeat, stirring gently, and cook until the soup is creamy--about 10-15 minutes. When ready to serve, adjust seasoning, stir in cognac, and ladle immediately into bowls. Lightly pinch a bit of nutmeg into each bowl.