For the salmon cakes: Drain the salmon, place it in a large bowl and flake the flesh with a fork. You should have about 2 cups. Add the onion, mayonnaise, breadcrumbs and mustard. Gently fold until well mixed, then season to taste with salt and pepper. Shape into six 1-inch-thick patties, each about 2 1/2 inches in diameter.
Heat a thin layer of oil in a large nonstick skillet over medium-high heat. Add the patties and cook, turning once, until browned, about 2 minutes per side. Cover and cook until heated through, about 2 minutes longer. Keep hot.
For the hollandaise sauce: Microwave the butter in a small microwave-safe bowl until melted and very hot, at least 30 to 40 seconds. In a blender or in the cup of an immersion blender, combine the egg yolks, lemon juice and mustard. Blend until the yolks are smooth and whipped.
With the blender running, slowly pour in the hot butter. Season the sauce with the cayenne and salt to taste. Keep warm until ready to use.
For assembly: Pour 2 inches of water into a large skillet and bring to a simmer over medium heat. Crack each egg into a small cup or bowl and gently tip it into the simmering water. Cook for 2 minutes, then turn off the heat and let the eggs sit in the hot water until the whites are set and the yolks just begin to firm, about 8 minutes. Remove each egg with a slotted spoon and allow to drain in the spoon over a paper towel to remove any excess water.
Place a hot salmon cake in the center of a plate, then top with a poached egg. Pour a generous tablespoon or two of warm hollandaise sauce over the egg, then sprinkle with the fresh chives and garnish each plate with a lemon wedge. Repeat with the remaining salmon cakes. Serve immediately.