For me, this recipe brings back fond memories of my trip to Normandy in France, where the cider is second to none. If you don’t have cider, use white wine rather than beer, which would be too strong. The granny smith apples work well due to their tart flavour as well as their firm texture, which doesn’t go too soft when cooked. The sorrel adds a lovely sharp flavour to the sauce. However, if you can’t find it, you can use baby English spinach leaves instead and add another shot of lemon juice to increase the acidity.