Heat a large pan over high heat. Add the butter, then the meat. Toss to coat meat, and saute just long enough to brown meat on all sides.
Remove from heat. Place meat in a large baking dish. Add carrots, potatoes and onion - and any other vegetables. Mix well.
Pour the beer and 1/2 of the water into a small saucepan. Bring to a simmer. Mix cornflour with remaining water until smooth. Slowly pour cornflour mixture into simmering beer mixture, whisking constantly. Continue to simmer until mixture has thickened.
Pour mixture over meat and vegetables.
Trim puff pastry to fit over top of filling.
Bake for 45 to 50 mins or until top is golden brown.
The sauce was quite tasty, but too thin. The vegetables would need to be cut up quite small to cook in this manner - mine were bite-sized but did not get soft enough in the recommended time. - 24 Jan 2012