2 Tbs. olive oil
12 oz. salted butter, divided
1 cup thinly sliced yellow onions
1/2 tsp. dried thyme
3/4 tsp. kosher salt
1/2 tsp. black pepper
4 cups chopped fresh summer tomatoes or 32 oz. canned diced tomatoes, not drained
3 Tbs. tomato paste
1/4 cup all-purpose flour
3¾ cups chicken broth, divided
1 tsp. sugar
1 cup heavy cream
Chopped fresh basil for garnish
Heat the oil and eight ounces of the butter in a Dutch oven over medium heat. Add the onion, thyme, salt, and pepper, and cook them until the onions are wilted, about eight minutes. Stir in the tomatoes and tomato paste and simmer for 10 minutes.
Place the flour in a small bowl and stir in five tablespoons of the chicken broth, blending them well to make a paste. Stir paste into the tomato mixture. Add remaining broth, stirring constantly to blend well, and simmer for 30 minutes, stirring frequently. Remove from heat and cool to room temperature. Wipe out the Dutch oven.
Pour the cooled mixture into a blender and process until smooth. (You may use an emulsion blender instead of a blender.) Strain the mixture through a fine-mesh strainer back into the Dutch oven. Add the sugar and heavy cream, bring to a simmer over medium heat, and simmer for five minutes, or until the bisque is warm. Stir in the remaining four ounces of butter. Serve with the chopped basil sprinkled on top.
The soup will keep for up to one week in the refrigerator or up to three months in the freezer. Freeze it prior to adding sugar and heavy cream; add them when reheating.