1. Heat 2 tablespoons butter in a medium saucepan. Add the onions, garlic,mustard and salt. Cook over low heat until the onions are translucent, about 15-20 minutes; don't let them or the garlic brown. This soup isn't supposed to taste brown.
2. Add the chicken stock and wine. Cover the an and let it simmer over low heat.
3. Meanwhile, in another medium saucepan, over medium heat, melt the remaining 3 tablespoons butter. Whisk in the flour all at once and cook it for 1 minute, whisking constantly. Then turn the heat way, way down and whisk in the milk. Cook this mixture, whisking constantly, until it makes a smooth, thick sauce. Add the cheese, kirsch and horseradish and whisk till combined. Now you have a cheese sauce. Add it to the onions and mix thoroughly. Add salt and pepper to taste, grate in a little nutmeg and cook the soup over low heat for 10 minutes. Stir it often. Then serve it.
Notes: From Ann Hodgman's BEAT THIS! COOKBOOK, 1993, Chapters Publishing Ltd., ISBN 1-881527-21-2, pages 141-142.