Tear up the white bread and pulse the pieces in a food processor to make fine, soft crumbs. (You should have about 6 cups crumbs.) Remove the bread crumbs to a shallow pan and mix in 1/2 cup of the chopped parsley (reserving the remaining 1/4 cup for the crabcake mixture). Set aside.
In a food processor, combine the egg yolk, lemon juice, Worcestershire, hot pepper, mustard, paprika, thyme, celery seeds, and black pepper and pulse to combine. With the motor running, slowly add the oil through the feed tube in a steady stream until the mixture emulsifies and forms a mayonnaise. Remove the mayonnaise from the food processor and refrigerate.
Place the crabmeat in a cheesecloth-lined sieve set over a bowl. Pull the cheesecloth tightly around the crabmeat and squeeze out as much juice as possible. Place the chopped onions and bell peppers in a sieve set over a bowl and use your hands to squeeze out as much juice as possible. In a large bowl, combine the onions and bell peppers with remaining 1/4 cup parsley. Add the chilled mayonnaise and crabmeat and toss lightly to combine. Add 1 cup of the bread crumbs-parsley mixture and combine. Do not overwork the mixture or the crabcakes may get gummy. Gently form 8 patties and roll the patties lightly in the remaining bread crumb-parsley mixture. Leave the crabcakes in the pan of the bread crumbs until you saute them.
Preheat the oven to 425 degree. Using a nonstick saute pan and butter as needed, panfry the crabcakes, in batches, until golden brown on both sides and place them on a baking sheet as they are browned, put them in the oven until they are heated all the way through, 5 to 8 minutes.
Serve 1 crabcake as an appetizer or 2 as an entree. Serve with a ramekin of cocktail sauce and a lemon wedge.
Cook's Note: The crabcakes hold together better if prepared a day ahead and stored in the refrigerator before cooking. Store them in the pan of bread crumbs, covered with plastic wrap.
Green Cocktail Sauce:
Put the tomatillos in 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 small bowl and stir in the vinegar, sugar, pepper sauce, garlic, mustard seeds, and horseradish until combined.