1In a medium saucepan, melt butter on high. Sauté onion 1 min until tender. Stir in half flour and cook 1 min. Remove from heat. Gradually add milk, whisking until well combined. Return to heat and cook, whisking, until mixture boils and thickens. Simmer 1 min. Cool. Cover sauce surface with plastic wrap and chill 20 mins.
2Mix salmon, dill, zest and juice into sauce and season to taste. Cover and chill 1 hour, until firm.
3Form 2 tbsps mixture into fat oval shapes. Dust in remaining flour, shaking off excess. Dip into egg, then coat in crumbs. Chill 30 mins.
4In a deep saucepan, heat oil on medium until a few crumbs sizzle when added. Deep-fry croquettes in four batches, 3-4 mins, turning until golden and cooked through. Drain on paper towel. Sprinkle with dill and serve with lemon wedges and aioli.
TIP: Try double coating croquettes by dipping into egg and crumbs again.
Photo: Rob Shaw/Bauer