Begin by assembling all your ingredients.

In a medium-sized saucepan, melt 2 tablespoons of butter over moderate heat. Add the chopped onion, carrot, celery, and garlic. Cover and cook over medium-high heat, stirring intermittently, until the vegetables soften and begin to take on a light golden color, about 5 minutes.

Sprinkle the flour over the sautéed vegetables and stir continuously for a minute over low heat, ensuring the flour is fully integrated into the mixture.

Pour in the chicken stock, followed by the drained tomatoes, tomato paste, and sugar. Bring the mixture to a boil, then partially cover the saucepan and let it simmer on medium heat for about 15 minutes, stirring occasionally, until the vegetables are fully tender.

Carefully transfer half of the soup to a blender and blend until smooth and creamy. Pour the pureed soup back into the saucepan, stir in the heavy cream, and continue to heat until the soup is thoroughly warmed through.

Season the bisque with kosher salt and freshly ground white pepper to taste. Swirl the remaining 2 tablespoons of butter into the soup just before serving. Ladle the bisque into bowls, top with croutons if desired, and serve hot.