Spiced flat fish with a red onion salad

4 flat fish fillets such as plaice, brill or sole

1 tsp pink or black peppercorns

1 tsp dried rosemary

3 tbsp rapeseed oil

1 tbsp lime juice

bunch of chives, finely chopped

bunch of parsley, finely chopped

1 small red onion, thinly sliced

3 tomatoes, diced