Carefully rinse the mackerel under running water then pat dry with kitchen towel.
Place a heavy-bottomed frying pan over a high heat. When hot, add the butter and when it is melted, place the fillets in, skin side down. Fry for 5 minutes on the skin side, then carefully turn the fillets. Cook for 2-3 minutes, reducing the heat a little to stop the flesh burning.
Toast the bread while the fish is cooking, then spread each slice with half the horseradish. Serve and enjoy.
Please note: this recipe has been created and uploaded by a member of the Tesco Real Food online community. Tesco does not test customer recipes so cannot accept responsibility for the content, validate the results or answer queries regarding them.