Set a box grater in a bowl, and grate tomatoes. Discard skin.
Melt butter in a saucepan over medium heat. Add onion and a pinch of salt. Cook for 3 minutes or until tender, stirring frequently. Stir in flour and cook for 1 minute, stirring constantly. Stir in tomato pulp and juice, stock, and a pinch of sugar. Cook for 5-10 minutes or until thick enough to coat the back of a spoon. The cooking time will depend on the size and juiciness of your tomatoes, so if it needs more time to thicken, go with it. Taste and adjust the salt, pepper, and sugar as needed. (Makes about 2½ cups.)