Heat olive oil in a large pot over medium heat. Sauté diced onions until translucent, about 5 minutes. Add minced garlic and cook until fragrant.

Stir in chopped tomatoes and vegetable broth. Bring to a boil, then reduce heat and simmer for 20 minutes.

Blend the soup until smooth using an immersion blender. If desired, leave some chunks for texture.

Stir in heavy cream over low heat and season with salt and pepper.

Butter one side of each slice of bread, place cheese between unbuttered sides, and grill on medium heat until golden brown and melted.

Serve soup hot alongside grilled cheese sandwiches cut into triangles.