makes 2 generous quarts
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 slice the onions.
Heat olive oil and butter in a soup pot over moderate heat on the stovetop. Add the sliced leeks and onions. Cook about 10 minutes without browning, reducing temperature if necessary, covering pot to sweat, giving a stir now and then. Add the thyme about half way through this time.
Sprinkle in the sugar and salt and stir. Add the tomatoes, 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.
Pass the soup through a food mill or chinois into a large bowl then transfer back to the pot, add the cream (up to 1 cup) and heat gently without boiling, which will curdle the cream.
Alternatively, you can use a blender, but this will make a thicker-textured soup as the onions and leeks will be puréed instead of extracted as with the other technique. Both methods make good soup.