Rub the inside of a cheese fondue pot or medium enameled cast-iron casserole with the garlic clove; discard the garlic. Combine the grated Gruyère and Emmentaler with the wine, cornstarch and lemon juice in the fondue pot and cook over moderate heat, stirring occasionally, until the cheeses begin to melt, about 5 minutes. Add the kirsch and a generous pinch each of pepper and nutmeg and cook, stirring gently, until creamy and smooth, about 10 minutes; don't overcook the fondue or it will get stringy. Serve at once.