In a large bowl, combine the mayonnaise, onion, eggs, Worcestershire sauce, dry mustard, salt and cayenne. Fold in the crabmeat and 1/4 cup of the cracker crumbs. Shape the mixture into 16 cakes about 1-inch thick. Coat the crab cakes with the remaining cracker crumbs and transfer to a baking sheet lined with waxed paper. The crab cakes can be refrigerated overnight at this point, if needed. In a large skillet, melt 1 tablespoon of the butter in 2 tablespoons of the oil. When the foam subsides, add half the crab cakes and cook over moderate heat until golden and crisp, 2 to 3 minutes per side. Drain the crab cakes on paper towels, then keep warm in a low oven. Repeat with the remaining 1 tablespoon of butter and 2 tablespoons of oil and cook the remaining crab cakes. Serve with lemon wedges.