1. Heat olive oil in a large pot over medium heat. Sauté the onion and garlic until soft and fragrant.
2. Add the tomatoes and cook for 5 minutes. Pour in the vegetable stock, bring to a boil, and then simmer for 15 minutes.
3. Remove from heat and blend until smooth using an immersion blender or stand blender.
4. Stir in the cream, basil, salt, and pepper. Simmer for another 5 minutes.
5. While the soup cooks, heat a skillet on medium heat. Butter one side of each bread slice.
6. Place a slice of cheddar between two pieces of bread (buttered side out) and grill until golden and melty, about 3 minutes per side.
7. Serve the soup hot with a swirl of cream and fresh basil, and pair with the grilled cheese.