Put the egg yolk, seafood seasoning, mustard, lemon zest and juice, and vinegar in the bowl of a food processor or a blender and process until smooth. Gradually pour in the oil with the machine running until the mixture emulsifies and forms a mayonnaise. Season with the salt and pepper. (Note: this quantity of mayonnaise will not emulsify in the bowl of the largest size food processor. Use a medium size food processor, a mini processor, or a blender.)
Transfer the mayonnaise to a bowl and, using a rubber spatula, fold in the scallions and the crabmeat until well combined. Combine the bread crumbs and the parsley in a shallow container. Form the crab mixture into 8 patties about 3-inches wide and 3/4-inch thick and drop them into the bread crumb-parsley mixture. Dredge the crab cakes on both sides. If you have time, leave the crab cakes in the container of bread crumbs, cover with plastic wrap, and chill for 1 hour or more.
When you are ready to fry the crab cakes, put 2 large nonstick skillets over medium heat. Add about 2 tablespoons butter to each pan. When the butter is melted, add 4 crab cakes to each pan, patting off excess crumbs first. Slowly fry the crab cakes over medium heat until they are golden brown on both sides and hot through, turning once with a spatula, about 4 minutes per side. If the crab cakes are browning too quickly, reduce the heat. The internal temperature of a cooked crab cake should be 155 degrees F on an instant-read thermometer.
Transfer the crab cakes to plates, 2 per person, and serve with your choice of sauce and lemon wedges.
Green Cocktail Sauce:
8 ounces tomatillos, husked and cut into quarters
2 tablespoons rice wine vinegar
1 tablespoon sugar
2 teaspoons green hot sauce (recommended: Tabasco)
1 teaspoon chopped garlic
1 teaspoon mustard seeds, toasted
1 teaspoon peeled and grated fresh horseradish
Put the tomatillos in the bowl of a food processor and process until coarsely pureed. Remove the tomatillo puree to a sieve, drain off the liquid and discard. Put the drained puree in a bowl and stir in the vinegar, sugar, hot sauce, garlic, mustard seeds, and horseradish.