In a medium pot, combine crushed tomatoes, vegetable broth, garlic, onion powder, and basil. Bring to a simmer over medium heat and cook for 15 minutes.
Use an immersion blender to puree the soup until smooth (or carefully transfer to a regular blender in batches). Stir in the milk and season with salt and pepper. Keep warm on low heat.
While the soup simmers, assemble the sandwiches by placing 1 ounce of cheese between two slices of bread for each sandwich.
Heat a large nonstick skillet over medium heat and spray with cooking spray. Place sandwiches in the pan and cook for 3-4 minutes per side until golden brown and cheese is melted. Spray the top of each sandwich lightly before flipping.
Cut each grilled cheese in half and serve alongside a bowl of tomato soup for dunking.