Mist a nonstick frying pan with cooking spray and cook the mushrooms over a medium heat for 4-5 minutes until softened. Add the garlic and spinach and cook, stirring, until the spinach has wilted, then stir in the tarragon and mustard. Season and transfer to a small bowl.

Wipe the pan clean. Lightly whisk the eggs and milk together in a small bowl. Mist the pan with more cooking spray and return to the heat, then pour in the egg mixture and tilt the pan so the base is covered. Leave for a minute to allow the eggs to set, then push the cooked edges into the centre using a spatula, letting any uncooked egg run into the gaps.

Repeat until the omelette is almost fully set. Top one side ith the mushroom and spinach mixture, then fold the omelette over the toppings and cook for another minute. Serve garnished with salad leaves and pepper.