Brine the salmon, if desired. To do so, place the salmon in a glass baking dish. In a large glass measuring cup, mix together 6 cups water with 1/4 cup salt and 1/4 cup sugar until both dissolve. Pour over the salmon pieces, making sure there's enough liquid to completely cover them. If there isn't, add more. Cover with plastic wrap and refrigerate at least 1 hour and up to 3 hours. When ready to cook, remove salmon from brine, discard brine and rinse and pat salmon dry.
To cook the salmon: Season the salmon pieces lightly with salt and black pepper.
In a small bowl, combine the ancho chili powder, paprika, garlic powder, cayenne and cumin. Sprinkle all over the salmon. Let stand for 15 minutes.
Meanwhile, in a separate small bowl combine the mustard and maple syrup.
In a blender, food processor or drink blender combine the avocado, sour cream, lime juice and pinch of salt. Process until the mixture is smooth, but slightly on the thick side, like mustard. Set aside.