Whisk together the egg, mayonnaise, rice wine vinegar, Worcestershire sauce, Chinese mustard, chopped parsley, salt and pepper. Stir in the Panko breadcrumbs. Gently fold in the crab (you want the crab to stay in big chunks). Taste and adjust seasonings as necessary (as Jeff says, try not to eat all the mix). Do Ahead: The mixture can be made up to 1 hour in advance and refrigerated (the cakes will take slightly longer to cook).
Preheat the broiler.
Melt 1 tablespoon of butter in a large nonstick or cast iron skillet over medium-high heat. As Jeff explains, “At this point, I grab a bunch of the crab mixture with my fingertips—a little more than the size of a golf ball—and place it into the pan, leaving ample space between each one.” You’ll be able to cook about a third to half of the mixture at a time, depending on the size of your pan. Fry the cakes until golden brown and crispy on the bottom, about 3-4 minutes. Carefully transfer the cakes to a baking sheet. Wipe out the skillet with paper towels, melt another tablespoon of butter, and repeat until all the crab mixture is used.
Place the baking sheet on the top rack under the broiler. Cook, watching closely, for 3-5 minutes or until the tops of the cakes are browned in spots. Serve immediately out of the oven with a sprinkle of fresh lemon juice and Sriracha Remoulade sauce.