For soup, remove crusts from 1-inch bread slices and discard. Tear bread into 1-inch pieces; set aside.

In a large pot heat 2 tablespoons oil over medium-high heat. Add onion, garlic and red pepper flakes. Cook 3 to 5 minutes or until onion is softened. Stir in tomatoes and basil. Using a potato masher, mash until tomatoes break down. Stir in brown sugar and bread pieces. Bring to boiling; reduce heat. Simmer, uncovered, for 10 minutes or until bread begins to break down.

Transfer cooked mixture in two batches to a blender or food processor; add 1 tablespoon oil to each batch. Cover and blend until smooth. Return soup to pot. Stir in chicken stock. Bring mixture to boiling; reduce heat. Stir in yogurt and simmer until heated through. Season to taste with salt and pepper. Serve warm topped with grilled cheese croutons.

For grilled cheese croutons, heat a large skillet or griddle to medium heat. Spread butter on one side of each ½-inch bread slice. Place two slices, buttered side down, in skillet. Top each with two cheese slices and remaining bread slices, buttered side up. Cook about 2 minutes or until bottoms are golden. Turn sandwiches over; cook 2 to 3 minutes more or until bottoms are golden and cheese melts. (Adjust the heat as necessary to prevent over-browning.) Cool slightly. Cut into 1-inch cubes.