Melt butter and olive oil in a 6-quart pot over medium heat.
Add diced onion and sauté for 5 minutes until translucent.
Add sliced garlic and cook for 1 minute.
Stir in tomato paste and flour, cooking for 2 minutes while stirring constantly.
Slowly pour in chicken stock while whisking to dissolve the flour and tomato paste.
Add canned tomatoes with juices, sugar, and thyme sprig.
Bring to a boil, then reduce heat to low and simmer uncovered for 30 minutes.
Remove from heat and discard thyme sprig.
Use an immersion blender to puree the soup until completely smooth.
Taste and season with salt and pepper.
Serve immediately.