1. To make the cakes: In a large mixing bowl, stir together the salmon, scallions, one of the eggs, the garlic, breadcrumbs, mayonnaise, whole-grain mustard, Dijon mustard, salt and onion powder. Divide the mixture into 4 larger or 6 smaller portions, and form the portions into cakes.
2. Place the flour, the remaining beaten egg and the panko in their own separate wide, shallow bowls. Dredge the cakes first in the flour, then in the egg and then in the panko mixture to coat.
3. Film the bottom of a large non-stick skillet with oil, and preheat the oil until shimmering. (Or, if you have a laser thermometer, to 375°F. No thermometer? When you place the edge of a cake into the skillet, you should hear a good sizzle—if you don’t, remove your cake and give it another minute to preheat.)
4. Fry the cakes on both sides until golden, about 3 to 4 minutes per side.
5. While the cakes are cooking, make the sauce by stirring all sauce ingredients together in a small bowl. Serve immediately.