Grate all the cheeses into a bowl and the mix with the cornstarch, coating the cheese well. Place the wine, garlic and lemon juice in the liner of the Instant Pot and turn on sauté, normal setting. When hot, add the cheese mixture and let melt all cheeses. Stir in the mustard and nutmeg, blending well. Serve with cubes of French bread, lightly steamed vegetables, hot chunks of sausage, or pieces of roasted potatoes.