2Place the plum tomatoes in a large baking dish. Drizzle with 2 tablespoons of the olive oil. Roast until the tomatoes are soft, 25 to 30 minutes.
3In a large stockpot set over medium heat, heat the remaining 2 tablespoons of olive oil. Add the onion and garlic and cook until the onion is soft, about 5 minutes.
4Add the roasted tomatoes and their juices, the canned tomatoes and their juices, and the stock, and bring to a boil. Reduce the heat to a simmer, stir in the cream, and season with salt and pepper. Simmer the soup for 10 minutes.
5Remove the pot from the heat and use a handheld blender to puree the soup until it’s as smooth or as chunky as you prefer (If you don’t have a handheld blender, you can use a potato masher to break up the tomatoes, or you can crush them against the side of the pot with a wooden spoon.)
6Make 6 sandwiches with 1 slice of cheese between slices of the white bread. Cut off the crusts and cut each sandwich into quarters. Crimp the edges of the bread together with a fork to make little packets that enclose the cheese. You should have 24 cheese dumplings.
7In a large skillet set over medium-high heat, melt a tablespoon of the butter. Working in batches and adding more butter as needed, grill the cheese dumplings until they are browned and toasted like little grilled-cheese sandwiches.
8Ladle the soup into 6 bowls and top with the dumplings. Serve immediately.