Place the crab meat in a large bowl. Be careful not to break up the lumps too much. Add the gluten free panko breadcrumbs, mayonnaise, egg, dijon mustard, lemon juice, old bay seasoning, parsley, green onions, and salt. Gently fold everything together with a spatula until just combined.
Divide the mixture into 6 equal portions. Shape each into a thick patty about 3 inches wide and 1 inch thick. Place them on a parchment-lined plate and refrigerate for at least 30 minutes to firm up.
Melt the butter in a large nonstick skillet over medium heat. Once the butter is sizzling, carefully place the crab cakes in the skillet, leaving space between each one.
Cook for 4 to 5 minutes per side without pressing down, until a deep golden crust forms on each side and the crab cakes are heated through.
Transfer to a plate and serve immediately with lemon wedges and your favorite dipping sauce.