How to Make It
Preheat the oven to 300°. Bring a large pot of water to a boil and fill a large bowl with ice water. Using a sharp knife, score an X on the bottom of each plum tomato. Blanch just until the skins start to peel back, 30 seconds. Using a slotted spoon, transfer the tomatoes to the ice bath to cool, then peel them.
Set the peeled tomatoes on a rimmed baking sheet. Drizzle with 2 tablespoons of the oil, sprinkle with the thyme and sugar and season with salt. Toss to coat. Roast the tomatoes for 1 hour, until very soft but not too shriveled. Let cool.
Meanwhile, in a skillet, heat the remaining 2 tablespoons of oil. Add the bread and toast over moderate heat until golden on both sides, about 3 minutes. Transfer to a plate and rub both sides with the garlic clove. Tear into bite-size croutons.
In a small saucepan, bring the stock to a light simmer. Place the cheese in a blender, add the stock and puree until completely smooth.
Spoon some of the warm cheese fondue onto plates. Top with the heirloom tomatoes, roasted tomatoes and croutons. Garnish with basil and sea salt and serve.