In a medium saucepan over medium-low heat, melt butter and stir in flour until mixed well. Add milk, half-n-half and mix well until no lumps remain.
Add potatoes, mustard, Zatarain's, salt and pepper. Bring to a boil, then reduce heat to a simmer and cook uncovered for about 20 minutes or until potatoes are cooked and sauce thickens.
Add sherry and crab and cook just a few minutes more to warm the crab.
Serve warm and garnish with chives and/or freshly ground pepper (both optional).