Preheat oven to 400°F. Roll out the prepared pie crust on a floured surface to about ¼” thick. Butter a pie plate to prevent sticking and place crust over the prepared pie plate. Poke holes in bottom with a fork to allow steam to escape. Bake pie crust for 10-15 minutes until middle is set. Reduce oven heat to 375°F.
Preheat cast iron skillet to medium-low heat, add the chopped bacon and cook until crispy, about 5 to 10 minutes. I like to use kitchen scissors, stack a few pieces of bacon at a time, and chop directly into skillet- quick & easy! Remove the bacon and place on a paper towel lined plate.
Add the onions and mushrooms (or other vegetable) to the same pan and sauté for 8 to 10 minutes in the bacon grease, until the onions are tender and browned. Add the greens & minced garlic to the pan and sauté an additional 1 to 2 minutes, until greens wilt and garlic is fragrant but not yet browned. Remove everything from the pan to a bowl or plate to cool.
Meanwhile, in a large bowl, whisk the eggs, milk, salt, and black pepper until smooth.
Add the bacon and sautéed onions, mushrooms and greens mixture to the large bowl of eggs and milk. Then, stir in half of the shredded cheese (about half a cup). Pour mixture into the prepared pie plate, then top with the remaining shredded cheese.
Bake at 375°F for 35 minutes, until the center is set and lightly golden on top. If the middle is still jiggly, bake another 3 to 5 minutes. Enjoy warm!