In a large pot, melt 2 tablespoons butter over medium heat. Add chopped onion and cook until translucent, about 5 minutes. Stir in flour and cook for 1 minute, stirring constantly to form a roux.

Slowly pour in chicken stock while stirring continuously to prevent lumps. Add canned tomatoes with their liquid and stir until smooth. Bring to a boil, then reduce heat to a simmer. Season with sugar, salt, and black pepper.

Optional: For smooth soup, divide soup into two portions and puree each separately in a blender until smooth. Return pureed soup to pot and bring back to a simmer.

Prepare cheddar bay biscuit mix according to package directions. Drop spoonfuls of batter into the simmering soup until all batter is used. Cover pot and cook for 15-20 minutes without lifting lid, until dumplings are cooked through and no longer doughy.

Remove pot from heat. Melt remaining 2 tablespoons butter and mix with the seasoning packet provided in the biscuit mix. Brush this mixture over the tops of the dumplings. Serve hot.