Beat the eggs, milk, herbs and salt and pepper together in a large bowl. Melt a quarter of the butter in an omelette pan. As soon as it stops foaming, swirl in a quarter of the egg mixture and cook over a medium heat, forking over the omelette so that it cooks evenly.
As soon as it is set on the underside, but still a little runny in the centre, scatter a quarter of the cheese and a quarter of the rocket leaves over one half of the omelette. Carefully slide the omelette on to a warmed serving plate, folding it in half as you go. For the best results, serve immediately, then repeat to make 3 more omelettes and serve each individually. Alternatively, keep warm in a moderate oven and serve altogether.