In a pot over medium heat, melt the butter and add the flour. Stir the flour until it is lightly toasted but does not change color.

Add the chopped onion and sauté briefly.

Add the tomatoes.

Pour in the water or broth.

Let it cook until the tomatoes are soft, about 10 minutes.

Blend the mixture in a blender until smooth.

Strain the soup through a fine mesh sieve.

Return the soup to the pot, add the sugar and vinegar, season with salt and black pepper, and bring to a boil.