1. In a blender, pulse 2 bread slices to make crumbs; place on a large plate. Cut remaining bread into 1/4 inch cubes.
2. In a bowl, toss crab with mayo, mustard, salt, pepper, lemon juice, and bread cubes just until combined. Cover and refrigerate 1 hour*.
3. In nonstick 12-inch skillet, melt butter over medium heat until browned. Meanwhile, scoop crab mixture by scant 1/2 cups and shape each into 3-inch patty; coat lightly with bread crumbs. Add crab cakes to skillet; cook 8 to 10 minutes or until golden on both sides and heated through, turning over once. Serve with lemon wedges.
*Be sure to chill for the 1 hour noted. Otherwise the crab cakes will fall apart in the pan.