Melt butter in a sauté pan over medium-high heat. Add onions and garlic and cook until softened and fragrant, about 2 minutes; scrape pan mixture into a large mixing bowl and cool for a few minutes. 

Add crabmeat, panko, garlic, mustard, Creole seasoning, and salt and pepper and combine; taste and adjust seasoning. Whisk one of the eggs and add to crabmeat mixture. Divide into 12 equal portions and shape into a ball around the meaty end of each crab finger, leaving the claw exposed. 

Whisk the remaining egg and buttermilk in a shallow bowl; spread the remaining panko on a plate. Gently roll each shaped crab finger in the egg mixture then the panko; place on a wire rack on a baking sheet. Refrigerate for at least 30 minutes and up to 1 day. 

Pour about 2 inches of oil into a deep pot and attach a candy thermometer; place over high heat and bring to 360ºF. Working in batches, fry crab fingers, turning often, until golden brown and drain on paper towels; return oil to temperature between batches if necessary. Serve hot with sauce as desired.

 Note: If you don't have access to crab fingers, shape the mixture in small patties or balls and fry as directed.