Remove beef and gravy from packet and place into an ovenproof dish. Cover with foil and cook for 30 minutes.
Meanwhile, place potato into a small saucepan and cover with cold water. Bring to the boil over high heat. Reduce heat to medium-high and simmer, partly covered, for 12 minutes or until potato is tender. Drain, return to pan and shake gently for 2 minutes until dry. Remove from heat and mash. Add milk, butter and parsley, then season with salt and pepper. Beat until creamy.
Trim sugar snap peas and place into a heatproof bowl. Pour over boiling water and leave to stand for 3 minutes or until bright green. Drain. Serve beef with mash and peas.
