Drizzle the cod fillets with lemon juice, season with salt and pepper.
Peel the onion and dice very finely. Let it become translucent in the clarified butter in a pan. Put in the cod fillets and fry for 1/2 minute on each side.
Reduce the heat, pour the wine into the pan. Cover the fish and cook for 4-5 minutes (depending on the thickness of the fillets) over medium heat.
Remove the fillets and keep them warm. Add the mustard and cream to the pan and stir. Simmer over high heat while stirring until slightly creamy.
Drain the capers in a small sieve, add to the sauce, season with salt and pepper. Place the cod fillets in the sauce and garnish with lemon and dill.