Chop the salmon into 1/2 inch to 1 inch chunks. Add half of the salmon to a food processor with all of the dijon mustard. Process the mixture until it becomes paste-like. This will act as a binder for your burger.
Add the remaining salmon to the paste as well as the diced onions, half of the capers, ginger, and lemon zest. Pulse the processor until the salmon chunks are coarse, but well mixed into the paste. Be cautious not the puree too finely or the result will be an easily overcooked burger.
Place the mixture in a bowl, mix in the breadcrumbs, thyme, and the rest of the capers. Salt and pepper to taste. Mold into four equal sized burgers. Refrigerate for at least one hour, and up to 24 to allow the burgers to firm up.
Heat a skillet over medium-high heat. Add olive oil. Carefully place the burgers into the skillet and brown for 3-4 minutes on each side. Meanwhile, mix the mayonnaise and sriracha in a bowl, and toast the burger buns (optional).
Serve the burgers with the sriracha mayo and lettuce. Enjoy.