Wash the tomatoes.
Heat a pan.
Place the tomatoes in the pan.
Add 2 to 3 cups of water.
Cook covered for 8 to 10 minutes on medium flame.
Turn off the heat and allow the tomatoes to cool.
Peel the tomatoes and blend.
Keep the tomato puree aside.
Heat 1 tsp oil in a pan.
Add mustard seeds.
When the seeds start spluttering, add hing, chopped garlic, chopped ginger, curry leaves, chopped green chillies and saute for few seconds.
Add chopped onion.
Saute for few minutes till the onions turn translucent.
Add tomato puree.
Check the consistency and add water if needed. Tomato saar has thin, soupy consistency.
Add turmeric powder and salt. Give a good mix.
Close the lid and cook covered for 3 to 4 minutes on low flame.
Next, turn off the heat and garnish with chopped coriander leaves.
Serve hot.