Split the English muffins with a fork, and spread 3 Tbsp softened butter on them. Grease a 2 quart rectangular baking dish and set is half of the muffins, buttered side up. (A two-by-three array works well.)
In a separate bow, fold together crabmeat,cheeses, and capers. Evenly distribute crab-cheese mixture on top of the muffins.
Put remaining muffin halves over mixture to cover (buttered side up).
In a pan, sautee onion in the remaining 1 Tbsp of butter for 4-6 minutes until cooked.
Add the sherry and the Worcestershire sauce to the onion and bring to a boil. Remove from heat.
In a separate bowl stir together the eggs, milk, mustard, parsley, and salt and pepper to taste (go easy on the salt due to the cheese and capers). Put the egg mixture over the muffins and press down on the muffins to help the egg mixture moisten them.
Cover and put in the refrigerator overnight.
The next day, preheat the oven to 350 degrees F. Bake uncovered for 50-60 minutes or until a knife inserted near the middle comes out clean. (It may look wet due to the oils, but you should not see any raw egg.) Let stand a few minutes before serving.