1. In a 1 1/2- to 2-quart fondue pan (flame-proof ceramic or porcelain-glazed cast-iron) or heavy-bottom metal pan over medium heat, warm 1 cup wine until bubbles form and slowly rise to surface, about 6 minutes.
2. In a bowl, mix Swiss cheese, gruyère cheese, cornstarch, and mustard.
3. Add cheese mixture, a handful at a time, to hot wine, stirring until fondue is smoothly melted and beginning to bubble. Add kirsch and sprinkle fondue with nutmeg and pepper.
4. Set pan over an ignited alcohol or canned solid-fuel flame (if pan is ceramic, place a heat diffuser between it and heat source). Adjust heat so fondue bubbles very slowly. Check occasionally to be sure fondue is not scorching; if it is too hot, reduce or turn off the heat, then resume heating when mixture begins to cool.
5. Spear bread cubes, 1 at a time, with fondue forks or thin skewers (metal or wood) and swirl through fondue (stir across bottom frequently to prevent scorching); lift out and let drip briefly over pan, then eat. If fondue gets too thick for easy dipping, stir in more heated wine, a few tablespoons at a time. After fondue is consumed, scrape the cheese crust from pan to divide and eat; it's considered a special treat.
Notes: If you use an electric fondue pan, turn heat to medium while mixing and melting fondue, then turn to lowest setting while serving.