To make the spice paste, heat a dry frying pan over a medium heat, then add the seeds and peppercorns and toast, shaking from time to time, for 2–3 minutes until the mustard seeds are popping and the spices are aromatic. You may need to cover the frying pan to stop the mustard seeds jumping.
Blend the toasted spices with the chilli to a smooth powder in a coffee grinder or with a pestle and mortar. Combine the powder with salt and the brown sugar. Store in an airtight container until needed.
Half an hour before cooking the fish, lightly spread both sides of each trout with the spice mixture – about 1 teaspoon per fish. Store any remaining spice mixture for future use.
Cook on a hot griddle pan for 3 - 4 minutes each side until lightly charred and firm to the touch.
For the cucumber pickle:
Put the cucumber in a colander. Sprinkle with the salt, vinegar and the sugar. Leave to stand for 30 minutes.
Lightly rinse with cold water. Drain, pressing out excess liquid with the back of a spoon.
In a medium-sized bowl, beat the soured cream with the mustard, then stir in the shallot, dill and cucumber. Cover and chill until ready to serve.
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