If using fresh salmon: put 1/4 cup water in a skillet with a little salt. Add salmon fillets skin-side down and cover. Steam over medium-high heat for about 5-7 minutes or until flaky; cool, remove skin and break it up a bit. If using canned salmon: drain the salmon well.
Stab the sweet potato a few times with a knife tip and microwave 4 minutes or until soft. Let cool until you can handle it and then peel.
While those cook, zest about 1/4 teaspoon lemon zest into a large bowl. Squeeze 1 teaspoon juice (reserve the rest for later) and add it to the bowl.
Add the sweet potato, mustard, salt and pepper and mash well.
Gently stir in the flaked salmon. Taste for salt.
Form into 4-8 patties and roll in breadcrumbs if you like. Cover and refrigerate until ready to cook.
Heat the oil or melt the butter over medium heat. Cook the patties slowly to brown well. Turn and cook the other side.
Serve with avocado sauce: Mash the avocado up with the chives and 1-2 teaspoons of lemon juice. Salt to taste and serve with salmon croquettes.