While the steak is resting crack the egg on to a saucer to make it easier to slide into the pan. Heat the butter in a heavy-based frying pan over a low heat, and find a slightly domed saucepan lid, ideally slightly smaller than the pan itself, so you can place it over the cooking eggs. Once the butter has melted, but not begun to foam, swirl it around the pan to coat, and then slide in the egg. If you’re cooking more than one, be careful not to crowd the pan. Cover and leave for 2 minutes, then check the white is cooked, lift out, season gently, and serve immediately.