Preheat the grill: Turn your grill to medium so it is ready when you need it.
Beat the eggs: Whisk the eggs and cream (if using) together in a small bowl.
Cook the mushrooms: Melt the butter in a 20cm (8 inch) grill-proof nonstick frying pan over a medium-high heat. Add the mushrooms and sauté until nicely coloured, about 4 minutes.
Add the eggs: Reduce the heat slightly, pour in the egg mixture, then quickly fold in the Gruyère cheese. Bring the raw mixture in from the sides of the pan. Season with salt and pepper, then sprinkle over the parsley.
Set on the hob: Cook for a couple of minutes without stirring until the omelette is lightly browned underneath.
Grill to finish: Place the pan under the hot grill for about 2 minutes until golden and puffed up.
Serve immediately: Transfer to a plate and serve at once with the dressed rocket alongside.
