In a medium skillet cook onion in butter for 3 to 5 minutes or until tender. In a large mixing bowl combine onion, carrots, 1/4 cup of the cracker crumbs, the egg, mayonnaise, mustard, and Old Bay seasoning. Add crabmeat. Use hands to mix well.
Line a baking sheet with waxed paper; set aside. In a shallow dish or pie plate combine finely chopped nuts and remaining 1/4 cup cracker crumbs. Mound about 1/3 cup of the crab mixture on top of the cracker mixture. Flatten and turn the crab cake, coating both sides with the cracker mixture. Place crab cake on the prepared baking sheet. Repeat with remaining crab mixture and crumb mixture. Cover crab cakes with another piece of waxed paper; chill in refrigerator for 1 to 2 hours.
In a large skillet heat peanut oil until hot. Cook crab cakes, half at a time, over medium heat for 6 to 8 minutes or until golden brown, turning halfway through cooking. Add additional oil during cooking, if necessary. Drain on paper towels. Makes 10.