Serving Suggestions: Toasted brown bread, mango chutney, and crumbled cooked streaky bacon
1) In a large, non-reactive, saucepan over medium-low heat, melt the butter. Add the onions, cook until softened but not browned, about ten minutes. Add the apple and continue cooking until cooked, but still slightly firm, about ten minutes more.
2) Sift the flour over the onions. Add the nutmeg. Cook and stir with a wooden spoon until the mixture is lightly browned and fragrant, about four to five minutes. Add the Calvados, if using, and cook to reduce by three-quarters.
3) Gradually whisk in the stock until well blended. Bring to a boil, reduce heat, cover, and simmer, stirring occasionally for ten minutes. Add the milk, return to just a boil, whisk in cheeses until completely melted. Season with pepper to taste.
4) Serve hot, with crumbled streaky bacon on top, and a crusty bread and chutney on the side.