In a 6-quart Dutch oven or soup pot set over medium heat, melt 2 tablespoons of the butter.  Add the chopped onion, celery, and garlic, stir to combine.  Cook the vegetables, stirring occasionally, until they are soft but not browned, about 5 minutes.

Reduce the heat to low, and add the remaining 2 tablespoons of butter and when it has melted sprinkle the flour over the vegetables and stir, cooking for 1-2 minutes to incorporate the flour. 

Add the chicken broth, tomatoes, tomato paste, dried thyme, and sugar.  Increase the heat to medium and bring to a boil.  Partially cover the pot and cook the soup, stirring occasionally until the vegetables are tender, for 15 minutes.

Scoop half of the soup into another pot and reserve.  Using an immersion blender or a regular blender, purée the remaining soup.  Return the reserved portion of soup to the pot, adjust the heat and bring to a boil, then reduce to low and stir in the remaining butter, salt/pepper and the heavy cream. 

Ladle the soup into soup bowls, garnish with a drizzle of extra-virgin olive oil and croutons.