In a heavy mortar, crush the pepper, mustard seeds, garlic and cumin seeds with a little salt, until the spices are fairly fine but not a powder and the garlic forms a paste.
On medium heat in a pot, fry the onions until translucent.
Add the crushed spices, garlic and red chillis. Fry for a minute, stirring. Add the turmeric and stir for 10 seconds.
Add the tomato and the tamarind juice (strain out any pips).
Bring to the boil and lower heat to medium. Cook for 15 minutes.
Add half a cup water (or more if the soup is too strong). Season with salt and serve with fresh coriander.
Serve over basmati rice, or strain and serve in mugs.
