Combine the tomatoes, slices of onion, a few garlic cloves, and chicken broth in a stockpot over moderate heat. To mix the flavors, bring it to a gentle boil and cook for 20 minutes.

Remove it from the heat and pour the mixture into a big bowl or pan after passing it through a blender or food processor.

Melt the butter in the now-empty stockpot over medium heat. Make a roux by stirring in the flour and heating it until it reaches a medium brown color.

Stir in the remaining tomato mixture after gradually whisking in small amounts to prevent lumps. Add salt and sugar, and season to taste.

Serve hot or cold with your desired bread or croutons.