Preheat the oven to 400 degrees F. For the crust: Use a 9-inch whole wheat ready-made crust.
Preparing 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 begin to brown, about 5 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 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.
For the filling:
4 teaspoons olive oil
1 large onion, sliced thinly into half moons
8 ounces sliced mixed mushrooms, such as cremini, oyster, shiitake
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)