Melt 1 tablespoon of the butter in a small saucepan over medium heat. Stir in 1 tablespoon of the flour and cook, stirring constantly, until the flour is dissolved, about 1 minute. Whisk in the cream and the nutmeg and increase the temperature to medium high. When the mixture starts to boil, reduce the heat to medium and, stirring frequently, cook the sauce until it is very thick, about 3 minutes. Transfer the sauce to a small metal bowl, press plastic wrap directly onto the top of the sauce and refrigerate until cold, about 45 minutes. The plastic wrap will prevent a skin from forming on.
Rinse and dry the saucepan and heat over medium-low heat. Add the remaining tablespoon of butter, the onions and garlic and cook until the onions are translucent, about 3 minutes. Line a plate with a paper towel and spoon the onion mixture onto the towel to drain. Refrigerate until cold.
In a bowl, combine the sauce, the onion mixture, chives, lemon zest and cayenne until thoroughly blended. Using a large rubber spatula, gently fold in the salmon.
Shape the mixture into 12 quenelles or cylinders (a small ice cream scoop is perfect for portioning out pieces, a little larger than a golf ball). The shape is important for even cooking and maintaining a creamy center. Spread the croquettes on a sheet pan and refrigerate until very cold, about 1 hour.
Preheat the deep fryer to 350°F or, if you prefer to pan-fry, heat 1 inch of oil in a skillet over medium high heat to 350°F.
Spread the remaining flour in a shallow dish and whisk together with a big pinch of salt. Crack the eggs in another shallow dish and whisk together with another big pinch of salt. In a third shallow dish, whisk the panko and a big pinch of salt. Dip the croquettes in flour, then egg, then panko crumbs. Deep fry the croquettes until golden brown and delicious, about 4 minutes. If pan frying, cook 2 minutes, turn and cook another 2 minutes. Remove to a paper towel to drain.