- In medium saucepan over medium heat, melt butter. Add ½ cup flour and cook, whisking, for 5 minutes, or until smooth paste forms. Gradually whisk in milk. Bring to a boil, reduce heat to medium-low and simmer for about 8 minutes.
- Remove from heat and season with nutmeg. Transfer sauce to bowl and cool completely.
- Fold smoked trout, egg whites and lemon zest into cooled sauce. Season with salt and pepper. Transfer mixture to piping bag and keep chilled.
- Place remaining flour and breadcrumbs in two separate quarter sheet pans (see Note). Add beaten eggs to third shallow pan.
- Using a 5/8-inch round piping tip, pipe 10-inch logs of croquette mixture into flour in sheet tray. Cut logs into 5 cm (2 in) pieces and roll in flour to cover, shaping into cylinders. Dip in beaten eggs then roll in panko to coat. Transfer to clean sheet tray. Repeat to form remaining croquettes. Refrigerate until cold. Repeat coating croquettas in egg and panko.
- In large pot, bring oil to 190°C (375°F). Working in batches, fry croquettas for about 1 minute, or until golden brown. Remove from oil and drain on paper towels. Sprinkle with salt and serve.
• To make ahead: Rolled and breaded croquettes can be held in freezer. Cook from frozen, adding 1 minute cooking time.
• Quarter sheet pans are flat, rimmed 23 cm x 33 cm baking sheets; any similar-sized shallow baking tray or slice tray will do.