Spray a 10-inch tart pan with cooking spray. Set aside.
Preheat oven to 350 degrees F (175 degrees C).
Place rolled oats in a food processor and mix until it turns into flour. It’s okay if the flour is still a little gritty. Pour in a large bowl. Mix in almond flour, oregano, cumin and salt. Whisk egg and egg white together, and mix with flours. A dough should start to form. Add water to the dough, about 1/2 tablespoon at a time. Use as much water as necessary to get a slightly moist dough. Place dough in tart pan, and using your fingers, press dough so that it covers every part of the pan. Be sure to press down along the bottom crease of the pan to prevent an overly thick crust. Bake for 15 minutes.
While dough is baking, prepare filling. Heat oil in a skillet over medium-high heat. Cook onions until soft, about 2 to 3 minutes. Add pepper and cook for another minute. Turn off heat. Mix in diced tomatoes, 1 teaspoon oregano and 1/2 teaspoon cumin. Set aside.
Whisk 4 eggs and remaining yolk in a medium bowl, Mix in milk, cheese, salt, and some pepper.
Take crust out of the oven and raise the heat to 375 degrees F (190 degrees C). The crust may puff up a little, but just press it down with a spoon. Spoon half the vegetables on to baked crust. Mix rest of vegetables with egg mixture. Pour egg mixture into tart pan. Sprinkle cilantro on top.
Bake quiche for 23 to 25 minutes, until the center of the quiche is firm to touch. The filling may puff up at different spots when it comes out the oven, but it will deflate once it cools.