Melt butter in a large saucepan over medium heat.
Add onions and thyme; cook, stirring often, until onions are soft and translucent, about 10 minutes.
Add flour; stir constantly for 3 minutes.
Add tomatoes with juices; cook over medium heat, stirring occassionally, until thickened and flavors meld, about 30 minutes. Discard thyme.
Stir in scallions, cream, and 1/8 tsp. cayenne.
Season to taste with salt, pepper, and more cayenne, if desired.