Step 1Bring a small pot of water to boil. Make an "x" with a knife in the bottom of each tomato then add them, a few at a time, to the pot. Cook 30 seconds to 1 minute. Remove the tomatoes and place in a large bowl of ice water, then peel and core. Cut the tomatoes in half horizontally. Squeeze the seeds from the tomatoes into a strainer set over a large bowl. Press against the seeds with a spoon to extract the juice; add enough water to the juice to measure 1 cup. Set aside the juice and discard the seeds. Dice the tomatoes.
Step 2Melt the butter in a large saucepan over medium-low heat. Add the onion and cook until tender, 2 to 3 minutes. Stir in the tomatoes and reserved juice. Stir in the sugar, celery seeds and salt. Bring to a simmer. Cover and cook about 15 minutes, stirring occasionally.
Step 3Puree the soup in a food processor or blender until smooth. Season with the cracked pepper to taste. Pour into serving bowls and garnish each serving with a basil leaf.