Melt the butter in a large soup pot over medium heat.
Add the chopped onion and cook until soft and translucent, about 5 minutes.
Sprinkle the flour over the onions and stir continuously for 1 minute.
Slowly pour in the chicken stock while whisking to prevent lumps.
Add the diced tomatoes with their liquid, sugar, salt, and black pepper.
Bring the mixture to a gentle boil.
Reduce the heat and simmer for 15–20 minutes, stirring occasionally.
For a smoother texture, blend the soup using an immersion blender or carefully blend in batches.
Prepare the biscuit mix according to package directions.
Once the Tomato Soup with Cheddar Bay Dumplings base is simmering, drop spoonfuls of dough evenly across the surface.
Cover the pot with a tight-fitting lid and reduce heat to low.
Steam the dumplings for 15–20 minutes without lifting the lid.
Melt the remaining butter and combine it with the seasoning packet.
Brush the cooked dumplings generously with the garlic butter mixture.
Serve hot and enjoy immediately.