In a medium saucepan, put chopped tomatoes, pepper, cayenne powder, cumin powder, turmeric, coriander, salt, 4-5 cups water, and 1/2 Celsius tamarind juice extracted by soaking a small ball of tamarind (or you can use 2 teaspoons of tamarind paste).
Bring all to a boil and then reduce heat and cook on low 5-6 minutes.
In a separate sauce pan, heat the oil.
Add mustard seed and let it sputter a bit.
Add curry leaves, onion, garlic and red dried chilies.
Sauté for a few minutes.
Pour cooked pepper water into this and simmer for two minutes.