Preheat oven to 450 F.
Place walnuts, pepper, cumin and salt in food processor and process, using on-off pulses until mixture looks powdery but not damp. Transfer to a shallow pan.
Place pieces of fish in nut mixture, turning to coat evenly. Gently press mixture into fish. Set aside.
Film ovenproof skillet with oil and place over medium-high heat.
When very hot, add a piece of fish and sear about 2 minutes per side, until brown and crusty outside.
Transfer to oven for about 5 minutes, until fish is firm but not overcooked.