Wash and roughly chop tomatoes. Peel, wash and roughly chop ginger. Remove stems of red chillies. Wash and pat dry and break each into two. Wipe urad dal.
Heat oil and add mustard seeds and asafoetida. As the seeds begin to splutter, add red chillies and urad dal. Add tomatoes and ginger.
Cook on low heat till tomatoes begin to soften. Add salt to taste. Mix well and cool. Grind this mixture to a slightly coarse texture.