Drain the chopped tomatoes through a sieve placed over a bowl. Reserve about a quarter of the juice captured in the bowl, discarding the rest (or save to use in a stew or bolognese).
Place the roughly chopped chillies, shallots, ginger and garlic in a blender and blitz to form a smooth paste. Transfer to a heavy-bottomed saucepan along with the sugar, vinegar, mustard seeds and reserved tomato juice.
Bring the mixture to a boil over a medium heat then simmer until reduced by a third. Add in the reserved drained tomatoes and season with salt and pepper to taste.
Turn the heat down slightly and cook gently for about 1–1.5 hours until the chutney is thick and shiny. Allow to cool before serving. The chutney can be stored in a non-metallic container in the fridge for up to 1 month.
