Heat the olive oil in a large pot over medium heat. Add the diced onion and cook for 5 minutes until it softens. Stir in the minced garlic and cook for 1 minute more.

Pour in the diced tomatoes and vegetable broth. Bring the mixture to a simmer and cook for 15 minutes. Remove the pot from the heat and blend the soup until smooth using an immersion blender or regular blender.

Return the soup to low heat. Stir in the chopped basil and heavy cream. Season with salt and black pepper to taste and keep warm while you prepare the croutons.

Butter one side of each bread slice. Place two slices buttered side down in a skillet over medium heat. Top each with two slices of cheddar cheese and cover with the remaining bread slices, buttered side up. Cook for 3 to 4 minutes per side until the bread is golden and the cheese melts.

Remove the sandwiches from the skillet and cut them into small cubes to form croutons. Ladle the soup into bowls and scatter the grilled cheese croutons on top before serving.