Mushroom and Cheddar Fritatta with Rocket Side Salad

1. Preheat the oven grill and set the rack about 10cm to 12cm from the heat source. 
2. Heat the oil in large ovenproof nonstick skillet over a medium-high heat. Saute the mushrooms until browned, seasoning with salt and pepper. Add the onion and saute until tender.
3. Whisk the eggs, egg whites, salt and pepper in a medium bowl. Pour over the vegetables in the skillet and mix gently. Reduce the heat to medium-low, cover the skillet, and cook until the eggs are set around the edges, about 4 to 5 minutes. 
4. Sprinkle with cheese and then place the skillet under the grill and cook until the cheese is melted and just browned, about 2 minutes. Let the frittata stand 1 minute before cutting into 8 wedges. Serve with salad on the side.

 For the salad: 

1. Combine all ingredients in a large bowl. 
2. Toss to coat leaves with dressing and serve with the frittata.