Heat the butter in a pan and sauté the garlic. Add the mushrooms, sauté and season. Pour in the wine and reduce completely. Stir in the grapes and set aside.
Add the cheese and cornflour to the fondue pot, mix well. Pour in the wine. Bring to the boil while stirring, reduce the heat, continue to simmer until the cheese has melted. Stir in the Kirsch, continue to stir until the fondue is creamy, season. Top with the mushroom & grape mixture, garnish with thyme. Serve with bread.