In a large heavy-bottomed pot, melt 4 tablespoons of butter over medium heat.

Add the diced onion and sauté until translucent and soft, approximately 5 to 7 minutes.

Stir in the minced garlic and cook for 60 seconds until fragrant.

Whisk in the flour and tomato paste, cooking for 2 minutes to eliminate the raw flour taste.

Gradually whisk in the chicken stock and crushed tomatoes, ensuring no lumps remain from the roux.

Bring the mixture to a low boil, then reduce heat and simmer for 20 minutes.

While the soup simmers, prepare the grilled cheese by buttering one side of each bread slice.

Place cheese between the non-buttered sides of the bread and grill in a skillet over medium heat until golden brown on both sides.

Remove the grilled cheese from the heat and slice into 1-inch square croutons.

Use an immersion blender to process the tomato soup until completely smooth.

Stir in the heavy cream, sugar, salt, and pepper, then simmer for an additional 5 minutes.

Ladle the bisque into bowls, garnish with fresh basil, and top with the grilled cheese croutons immediately before serving.