The lemon rind garnish gives you a pop of flavor so you don't even need a sauce. If you don't love tarragon, use thyme.
Place bread pieces in a mini food processor; pulse until fine breadcrumbs form.
Finely shred lemon rind; set aside. Squeeze juice to equal 1 tablespoon. Combine juice, tarragon, salt, pepper, and egg in a medium bowl, stirring with a whisk. Add breadcrumbs; toss to combine. Add crabmeat; toss gently to combine. Shape crab mixture into 16 equal patties (about 1 1/2 tablespoons each); place on a plate. Chill 15 minutes to set.
Heat a large nonstick skillet over medium-high heat. Add oil; swirl to coat. Add crab cakes to pan; cook 2 minutes on each side or until browned and done. Arrange crab cakes on a platter; sprinkle evenly with rind.