Wash the haddock, dry, drizzle with lemon juice, salt and pepper, season with Cayenne pepper and flour. Pel the onion and dice finely. Melt the butter in a saucepan, fry the fish. Add diced onion, sauté until golden brown. After 10 minutes of cooking remove haddock from a saucepan, pour the broth, add watercress, bring to a boil and season.