Preheat your oven to 400°F (200°C).

Wash and thoroughly dry the fresh tomatoes, then slice them in half. Quarter the yellow onions and peel the garlic cloves.

Place the halved tomatoes, onions, and garlic onto a large unlined baking sheet. Drizzle generously with olive oil, season with salt and pepper, and toss with your hands until everything is evenly coated.

Roast in the oven for 45 minutes, or until the tomatoes and onions are soft and browned at the edges.

Heat a large Dutch oven or soup pot over medium heat. Transfer the roasted tomatoes, onions, garlic, and all accumulated pan juices into the pot.

Add the canned whole peeled tomatoes with their juices, chicken broth, fresh basil, crushed red pepper flakes, and a generous pinch of salt.

Stir well and bring the liquid to a boil. Once boiling, reduce the heat to low and let the soup simmer uncovered for 40 minutes.

Purée the soup directly in the pot with an immersion blender until completely smooth. Stir in the heavy cream, taste, and season with additional salt and pepper if needed. Keep warm on low heat.

To prepare the croutons, shred the cheddar and Gruyère. Spread softened butter generously over one side of each sourdough bread slice.

Heat a non-stick skillet over medium heat. Lay one slice of sourdough butter-side down in the pan, pile on the shredded cheese, and top with the second slice of bread butter-side up.

Cook for several minutes per side, flipping once, until the bread is crisp and deep golden brown and the cheese has completely melted.

Transfer the sandwich to a cutting board. Cut half into small, bite-sized crouton squares and slice the remainder into strips for dipping.

Ladle the hot soup into bowls. Swirl in a drizzle of heavy cream, top with the warm grilled cheese croutons, drizzle with olive oil, and finish with fresh basil leaves.