Pre-heat the oven to 220C/425F/Gas 7.
For the beef, heat the oil in a pan, add the onion and sauté for 1-2 minutes to soften.
Add the beef and cook for 3-4 minutes, until golden brown all over.
Pour the ale into the pan and then add the carrot and parsnip slices. Continue to cook for 5-6 minutes.
For the potatoes, heat the milk, cream and lemon zest in a pan until simmering.
Add the sliced potatoes and cook for 6-8 minutes, to soften.
Place the potato mixture into an ovenproof dish and place in the oven to bake for 8-10 minutes, until golden.
To serve, transfer the beef mixture to a serving plate and spoon the gratin potatoes alongside.