Cut the tomatoes in half and place them on a baking sheet. Drizzle with olive oil and season with salt and pepper. Roast for 30 minutes.
In a pot, heat some olive oil over medium heat. Add the onion and garlic, sauté until soft.
Add the roasted tomatoes and vegetable broth to the pot. Bring to a boil, then simmer for 20 minutes.
Blend the soup until smooth, then stir in the fresh basil.
For the grilled cheese, butter one side of each slice of bread. Place cheese between two slices, buttered side out, and grill until golden brown on both sides.
Serve the soup hot with grilled cheese sandwiches on the side.
Notes
For a creamier soup, add a splash of cream after blending.
You can substitute the vegetable broth with chicken broth if preferred.