1. Preheat the oven to 400 degrees. Coat a baking sheet with cooking spray. Place the tomatoes cut side up on the pan and roast 30 minutes.
2. Cut the garlic clove in half and rub the cut sides on the inside and bottom of a medium saucepan. Discard the garlic.
3. Cut the cheeses in ½-inch pieces and toss thoroughly with the cornstarch.
4. Combine the wine and sherry in the saucepan; bring to a boil and immediately reduce to medium heat. Add the cheese with any cornstarch in the bottom of the bowl and the nutmeg. Stir the mixture frequently until the cheese is melted. Keep warm over low heat.
5. Transfer to a fondue pot and keep warm.