In a medium bowl, combine salmon, mustard, parsley, cornichons, shallot, and panko crrumbs.  Stir well. Season with salt and pepper. Divide salmon into 4 equal portions and roll each into a ball, then flatten into a 1-inch thick burgers. Use your hands to smooth out any cracked edges  At this point you can cover and refrigerate until ready to cook.  Heat a large cast iron skillet over medium-high heat, add the oil and butter.  When hot, carefully place burgers into the pan and cook, carefully turning once or twice. - about 3 - 4  minutes per side.  Adjust heat lower to keep the oil sizzling but you do not want to burn the edges.  Serve with a wedge of lemon