Heat 1 tablespoon of the reserved bacon fat in a small saute pan over low heat. Add the onions and cook until translucent. Cool the onions for a bit.
Mix the bacon, onion, egg, mayonnaise, mustard, sugar, and lemon zest in a bowl. Add the salmon and potato, mixing gently after each addition. Form the mixture into 12 small patties. In a shallow dish, combine the bread crumbs, Parmesan, and pepper, to taste. Coat the patties in the bread crumb topping. Heat 1/4 cup of the oil in a large saute pan over medium heat, and cook the salmon cakes in batches until golden, about 3 to 4 minutes per side. Add more oil, as necessary.