How to Make It
Combine mustard, mayonnaise, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a shallow bowl. Spoon mustard mixture evenly over fillets. Combine panko, 1 1/2 teaspoons tarragon, and 1 teaspoon lemon rind in a bowl. Sprinkle panko mixture over fillets, pressing to adhere.
Heat 1 tablespoon oil in a large nonstick skillet over medium. Carefully add fillets, panko side down, to pan. Cook 3 to 4 minutes or until golden; turn and cook 3 to 4 minutes or until desired degree of doneness. Remove fillets from pan; keep warm.
Increase heat to medium-high. Add remaining 1 tablespoon oil to pan. Add snap peas and shallots; cook 3 minutes, stirring occasionally. Add remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, remaining 1 1/2 teaspoons tarragon, remaining 1 teaspoon lemon rind, and juice to pan; cook 2 minutes or until snap peas are crisp-tender. Serve with fillets.