Preheat oven to 350. Heat a skillet over medium-high and add the bacon, if using. Brown the bacon on both sides, then set aside on a paper towel to cool, reserving the pan drippings for cooking the veggies. If you are omitting the bacon, add the butter or ghee to the hot pan, then add the sliced leeks, asparagus, and mushrooms to the pan with the bacon fat or butter/ghee. Saute for 6-8 minutes or until veggies have softened. Season them with a bit of salt and pepper, then set them aside to cool. Meanwhile, whip the eggs and milk/cream/half & half in a small bowl until well incorporated, adding a dash of salt & pepper. Place the pie shell on a baking sheet and place the veggies into the pie shell in an even layer. Crumble the bacon over the veggies, sprinkle on the cheese, then pour the egg mixture over top. Give it a gentle shake to evenly distribute the liquid. Bake at 350 for 30 to 35 minutes, until set and just beginning to color on the top. Remove from oven and allow to sit for at least 10 minutes before serving.