Melt butter in a large heavy pot over medium heat. Add thyme, onion, and garlic. Cook until onion is completely soft and translucent. 10-12 minutes. Increase heat to medium-high; add tomoato paste. Continue cooking, stirring often, until paste has begun to caramelize in spots, 5-6 minutes.
Add tomatoes with juices, 1 tsp. sugar and 8 cups water to pot. Increase heat to high, bring to a simmer. Reduce heat to medium. Simmer until flavors meld and soup reduces to about 2 quarts (8 cups), 45-55 minutes. Remove soup from heat; let cool slightly. Discard thyme sprigs. Working in small batches, purée soup in a blender until smooth. Return to pot.
DO AHEAD: Soup can be made 2 days ahead. Let cool slightly; cover and chill. Rewarm before continuing.
Stir in 1/4 cup cream. Simmer soup until flavors meld, 10-15 minutes longer. Season to taste with salt, pepper, and remaining 1 tsp. sugar. Add more cream if desired.
Notes
For a lighter soup, omit the cream; or for a little decadence, add more cream or swirl in a little crème fraiche.