Heat a non stick frypan or wok over a medium to low heat. Once the pan is hot add the whole tomatoes to the dry pan. Roast them in the dry pan over a medium to low heat, turning as necessary, until the skins are slightly wrinkled and split.
Add the chilies to the dry pan and roast for a further few minutes.
Remove the tomatoes and chilis from the pan. Roughly chop the tomatoes and chop the chilis a bit more finely.
Heat the oil in the pan and add the fenugreek seeds, mustard seeds, ginger and garlic. Fry until the fenugreek seeds are just browning. Add the cumin, coriander, tomatoes and chillis.
Cook over a medium to low heat until there is no liquid left, and the achar is like a thick paste. Stir through the fresh coriander and serve.
You can also add lemon juice if you want to make it tangy.
Serve with rotti, bread or dolloped on your curries. Also great spread on burgers and wraps.