Whisk together mayonnaise and egg white in a medium bowl; add next 5 ingredients. Stir in crabmeat.
Shape mixture into 6 (4-inch) patties. (Mixture will be very soft.) Carefully dredge patties in breadcrumbs; cover and chill at least 1 1/2 hours.
Cook crab cakes in hot oil and butter in a large skillet over medium-high heat 3 to 4 minutes on each side or until golden. Serve immediately with Whole-Grain Mustard Sauce. Garnish, if desired.