Melt the butter in a large pot over medium heat. Add the chopped sweet onion and stir occasionally until soft and translucent.
Sprinkle in the flour over the softened onions and stir well to combine. Cook for about one minute.
Slowly pour in the chicken stock while stirring constantly. Add the canned diced tomatoes along with their liquid and stir everything together.
Add sugar, salt, and black pepper. Bring to a gentle simmer and let it cook for about 20 minutes.
Prepare the cheddar bay biscuit mix according to package instructions. Add melted butter.
Drop spoonfuls of the dumpling mixture into the simmering soup during its final minutes of cooking.
Ladle generous portions into bowls and serve.