Saute the onion and carrot in butter over medium heat until softened and beginning to brown (about 3-5 minutes).
Add chicken broth, tomatoes, salt, pepper, and 1/2 tablespoon thyme. Bring to a boil.
Use an immersion blender to blend soup in pot until smooth (alternatively, you can blend it in batches in a standing blender).
Turn heat down to a simmer and add spinach; stir until wilted (about 2 minutes).
Turn heat off and stir in the cream.
Serve garnished with remaining fresh thyme and a swirl of cream, if desired.