Drain the tuna and flake it into small pieces with a fork, smashing up any big chunks. In a medium bowl, mix it with the parsley, green onion, Old Bay, garlic powder, kosher salt, panko, Dijon mustard, mayonnaise, and eggs.
Form the dough into 8 patties using a 1/3 cup measuring cup to portion it out, then patting it into a patty with your hands. Refrigerate the patties for 15 minutes to firm up the texture (or up to 24 hours).
When ready to cook, heat 1 tablespoon olive oil in skillet over medium high heat. Add the patties and cook for about 4 to 5 minutes until lightly browned, then gently flip them with a spatula and cook about 3 to 4 minutes more until lightly browned on the other side. Repeat with the next batch, adjusting the heat because the pan will be hotter on the second batch (much like pancakes!).
Serve immediately, with a sauce and greens, or over rice, couscous, or quinoa. Leftovers store very well refrigerated for up to 3 days.