makes 3 quarts; use two and store one in the freezer for a nice surprise later! Adapted from Alice Waters and Ina Garten
Prep the leeks by removing the root end, slicing lengthwise, and then across every 1/2-inch or so. Place in a bowl of water and jiggle around to remove any dirt from between the layers. Lift out and transfer to a clean kitchen towel or paper towel to dry.
Peel and dice the onion. Remove the core from the fennel bulb (white part) and thinly slice the bulb, saving the fronds for garnish and the tender stem bases for another use.
Heat olive oil and butter in a soup pot over moderate heat on the stovetop. Add the sliced leeks, diced onion, and sliced fennel. Stir, lower heat, partially cover and cook until vegetables soften (but do not brown), stirring now and then. Add the thyme about half way through this time.
Sprinkle in the sugar and salt and stir. Add the tomato purée, water, and additional 2 tablespoons of butter. Bring to boil and then lower to maintain a simmer, keeping pot partially covered. Cook for 30 minutes to blend flavors. Remove from heat and let cool a bit before proceeding. Remove the thyme sprig.
Carefully ladle the cooled purée into a blender, processing half at a time if you have a good size Vitamix. (Be careful, don't fill your blender more than half way, partially vent the top, and go slow! Have a towel handy for any issues.) Alternatively, you can use an immersion blender in the pot.
Return blended soup to the pot, add the cream (up to 1 cup), and heat gently without boiling, which will curdle the cream.
Alternatively, transfer the blended soup into 3 quart jars and refrigerate until you are ready to heat and enjoy the soup. Add the cream just before heating and serving. (Tomato purée will last for several days in the refrigerator. It freezes well too, just leave ample space for expansion or use freezer bags.)
Serve with grilled cheese sticks or cut the sticks into croutons. I like to use sliced sourdough and TIllamook sharp white cheddar cheese. Butter the bread, sandwich the cheese, and heat until toasted and melty using a griddle, frying pan, or panini press.