Mushroom and Cheddar Fritatta and Arugula Side Salad

Ingredients:

• 3 tablespoons vegetable oil
• 1 (10-ounce) package mushrooms, chopped
• Salt and freshly ground black pepper
• 1 medium onion, chopped
• 8 eggs
• 4 egg whites
• 1 cup grated sharp Cheddar
• Arugula Salad, recipe follows

Source