Heat olive oil or butter in a large pot over medium heat.
Add the chopped onion and cook until softened, about 4 to 5 minutes.
Add the garlic and cook for about 30 seconds, just until fragrant.
Add the tomatoes, broth, salt, black pepper, and sugar if using.
Simmer for 15 to 20 minutes, stirring occasionally.
Blend the soup until smooth using an immersion blender, or carefully transfer it to a blender in batches.
Stir in the cream, milk, or half-and-half and warm gently.
Make a grilled cheese sandwich in a skillet with buttered bread and cheddar cheese.
Cook until both sides are golden and the cheese is melted.
Let the grilled cheese rest for 1 minute, then cut it into small squares.
Serve the tomato soup warm, topped with grilled cheese croutons.