Preheat oven to 400 degrees F (200 degrees C). Press pie crust into a 9-inch pie dish.
Heat a large skillet over medium-high heat. Cook and stir sausage in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease.
Heat oil in a skillet over medium heat; cook and stir onion in the hot oil until softened, 5 to 10 minutes.
Whisk eggs, milk, and mayonnaise together in a bowl until evenly mixed; stir sausage, onion, pepperjack cheese, mushrooms, and garlic into egg mixture. Pour mixture into the prepared pie crust; top with Cheddar cheese.
Bake in the preheated oven until a knife inserted into the center of the quiche comes out clean, about 45 minutes.
