Remove any bones from the salmon, dice it, and then finely chop it. Mix in the onion, if using it. Season lightly and shape into cakes.
Warm and split the buns, and mix the butter, lemon and mostarda, then butter the buns. In a non-stick frying pan, sear the burgers on both sides and place on the buns. Serve immediately. A fresh cucumber salad or salsa goes very well with this.