To make the crust, place the oats, flour, and salt in a food processor and pulse to combine. Add the butter and pulse about 12 times, until the mixture has a pebbly coarse texture. Add the buttermilk and pulse 3 to 5 times more to combine. Shape the dough into a flat disk, wrap it in a large piece of wax paper, and chill at least 1 hour and up to 2 days in advance.
When ready to bake, preheat the oven to 400°F and spray a 9-inch pie plate with cooking spray.
Open the wax paper so the dough sits on top of it. Place another large piece of wax paper on top of the dough. Roll the dough out, between the wax paper sheets, to an 11-inch circle. Remove the top sheet of wax paper. Transfer the crust, still on the other piece of wax paper, to the pie dish, then remove the wax paper from the top. Press the crust gently into the dish. Bake for 9 minutes, remove from the oven, and allow to cool. Reduce the oven temperature to 350°F.
1 medium onion, thinly sliced into half-moons
4 oz mixed mushroom, such as cremini, oyster, shiitake, coarsely chopped
½ tsp salt
¼ tsp freshly ground black pepper
¼ tsp dry mustard
1 Tbsp chopped fresh thyme
1 ½ cup cold low-fat (1%) milk
3 Tbsp all-purpose flour
2 large egg whites
3 large eggs
¾ cup grated Gruyère cheese
To make the filling, heat 2 teaspoons of the oil in a large nonstick pan over medium heat. Add the onion and cook, stirring occasionally, until softened, about 6 minutes. Reduce the heat to medium low, cover, and cook, stirring occasionally, until the onion is golden brown and caramelized, about 20 minutes more. Transfer the onion to a bowl. Add the remaining 2 teaspoons oil to the pan and heat over medium-high heat. Add the mushrooms and cook, stirring occasionally, until they have released their water and begun to brown, about 6 minutes. Add the onion back to the pan and stir in the salt, black pepper, mustard, and thyme.
In a medium bowl, whisk together the milk and flour until the flour is dissolved. Add the eggs and egg whites and whisk until well combined.
Sprinkle the cheese into the pie crust. Top with the mushroom-onion mixture. Pour the egg mixture on top. Bake until a knife inserted in the middle comes out clean, 45 to 50 minutes. Allow to cool for 5 minutes before cutting into 6 wedges and serving.