Melt butter in a small saucepan on medium heat. Add cream and simmer for 1 minute. Stir through lemon rind, juice and herbs and continue to simmer until reduced by one third, stirring constantly. Remove from heat and season to taste.
Cook bavette in a large saucepan of boiling, salted water according to packet directions. Drain and return to pan. Add cream sauce, rocket and smoked salmon, tossing carefully over a gentle heat to warm through. Serve.