Pre-heat oven to 210°C (fan)

Spray a tin with a decent amount of low calorie cooking spray and place in the oven until it starts foaming slightly. Probably 1 or 2 minutes

Put flour, eggs, milk and salt in a decent sized bowl and whisk, by hand, until the mixture is smooth

Pour the batter in the hot tin and place in the oven

Cook for 8 - 10 minutes, or until it has risen around the edges and is a golden colour. You don't want it too crispy as it may be more difficult to roll

While the Yorkshire is cooking, sauté the sliced mushrooms and onions in some low calorie cooking spray, until they are browned and cooked

Remove the Yorkshire pud from the tin and put the rocket inside

Add the beef, cooked onions and mushrooms (you can add some horseradish/mustard if you want, just check the nutritional values)

Roll up the Yorkshire pud and slice in half and serve