In a saucepan, combine chopped tomatoes, onion, ginger, and garlic. Cook over medium heat until softened.

Add sugar, vinegar, salt, mustard seeds, cumin seeds, and chili powder. Stir well and bring to a boil.

Reduce heat and simmer for about 30 minutes, stirring occasionally until thickened.

Remove from heat and let cool slightly. Blend until smooth if desired.

Store in sterilized jars and refrigerate.