Season fillets, cover them with a slice of onion and poach them until cooked, for 10 minutes approximately.
Combine yogurt, mustard, ají (chili) and lemon juice in a bowl. Heat over boiling water. Mixture must not boil. Season to taste. Add parsley.
Once fish is cooked, remove onion.
Cover fillets with yogurt and mustard sauce. Serve hot.
How to fillet, skin and debone fish