Position a baking sheet on the lower rack of the oven and preheat to 400 degrees . Using a fork, prick the inside of the pie shell all over. Place on the baking sheet and bake for 5 minutes. Arrange half of the cheese in the crust and bake until melted, about 3 minutes.
Meanwhile, in a large, heavy skillet, heat the olive oil over medium heat. Add the onion and leeks, season with salt and pepper and cook, stirring, for 3 minutes. Add the cauliflower and chicken broth and cook, stirring occasionally, for 10 minutes; let cool slightly.
In a large bowl, beat the egg whites and season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Stir in the remaining cheese and the cauliflower mixture. Transfer to the prepared pie shell and bake until the filling is set, about 20 minutes. Let cool slightly before serving.