Slice the tomatoes in half and peel and cut the garlic cloves. Drizzle a little olive oil onto the baking sheet then lay out the tomato halves and scatter on the garlic cloves and a pinch of salt and pepper. Place the tray into the oven and bake for 15 - 20 minutes.
In the meantime heat a large pot over medium heat. Add the rest of the olive oil and saute the onions, celery, and carrots for 5 minutes until softened.
Add the basil-roasted tomatoes, garlic, spices, salt and pepper, and stock, and allow to simmer for 20 minutes.
Use a stick blender to blend the soup until it's smooth.