Caramelize​d Onion, Chicken,Mushroom and Gruyere Quiche with Oat Crust

Quiche has always made my mouth water..drool drool !!!I was always very reluctant to make it on my own, thinking about the calories :).  Recently I visited my sister Sheeba at Abu dhabi and we were in a mall when my nephew Gabriel got his hunger pangs.We went to this quaint little coffee shop where they had chicken quiche as the featured item of the day. Wow!! You have to eat it to believe it but it was so good that I was instantly motivated to recreate the same culinary magic in my own kitchen some day soon.After coming back home, I went through many recipes online and amongst my 'secret' collections and here is one that seemed relatively healthy, calling for low fat or fat free ingredients.

Serves 6
Ingredients
For the crust:
Cooking spray
3/4 cup rolled oats
1/2 cup all-purpose flour
1/4 teaspoon salt
3 tablespoons cold, unsalted butter, cut into small pieces
3 tablespoons cold, lowfat buttermilk

For the filling:
4 teaspoons olive oil
1 large onion, sliced thinly into half moons
6 ounces sliced mixed mushrooms, such as cremini, oyster, shiitake
1/2 cup cooked chicken breast
1/2 teaspoon salt
1/4 teaspoon freshly ground black pepper
1/4 teaspoon dry mustard
1 tablespoon chopped fresh thyme leaves (or 1 teaspoon dry)
3 large eggs
3 egg whites
1 cup evaporated fat-free milk (not condensed milk)
2/3 cup grated Gruyere cheese (about 1-ounce)

Directions

• Preheat the oven to 400 degrees F. Spray a 9-inch pie dish with cooking spray.
• To prepare the crust, put the oats, flour and salt in the bowl of a food processor and pulse 3 times to combine. 
• Add the butter and pulse about 12 times, until you get a pebbly course texture. 
• Add the buttermilk and pulse 3 to 5 times more to combine. 
• Form the mixture into a ball and place it between 2 large pieces of waxed paper. 
• Roll out into a circle about 10 inches in diameter.
• Remove the top sheet of waxed paper. Transfer the crust, still on the other piece of waxed paper to the pie dish, then remove the waxed paper from the top. Press the crust gently into the dish. 
• Bake for 9 minutes, then let cool.
• To prepare the filling, heat 2 teaspoons of oil in a large nonstick pan over a medium-low heat. 
• Add the onion and cook, stirring occasionally, until golden brown and caramelized, about 20 minutes.
• Transfer the onions to a bowl. 
• Add the remaining 2 teaspoons of oil to the pan and heat over a medium-high heat. Add the mushrooms and cook, stirring occasionally, until they have released their water and begin to brown, about 6 minutes. 
• Add the onions back to the pan, stir in the salt, pepper, mustard and thyme.
• In a medium bowl whisk together the eggs, egg whites and evaporated milk.
• Sprinkle the cheese into the pie crust. 
• Top with the mushroom-onion mixture, chicken and pour the egg mixture on top. 
• Reduce oven temperature to 350 degrees F. Bake for 35 minutes or until knife inserted in the middle comes out clean. 
• Let stand for 5 minutes before serving.