Remove the skin from the salmon and ensure there are no bones. Cut into chunks, approx 2in/5cm cubes. Put the salmon, spinach, feta, lemon juice and a generous grind of pepper into the food processor and process until combined and broken up but try not to over-mix.
Remove the blade from the food processor then divide the mixture into 4 and form into burgers. They will be relatively soft - if you have time, I'd suggest you lay on a baking sheet/tray to firm up in the fridge for around an hour before cooking.
Towards the end of while they are chilling, make the sauce (or you can do during cooking if you prefer) - mash the avocado, mix in the lemon juice then the sour cream and garlic, if using.
Cook the salmon burgers either on the grill or in a skillet/frying pan with a little oil for approximately 3 minutes each side until gently browned and cooked through. Note they will cook quicker than a solid piece of salmon fillet. You'll see white-looking juices coming out the side as they are near done on each side.
Toast the buns then spread on a thin layer of the avocado cream on the bottom before topping with the burger.
Video
Notes
If you prefer, you can make these into smaller burgers to serve more as fishcakes with eg salad or grilled vegetables rather than as a burger, but they are tasty either way!