Preheat the oven to 450 degrees F (230 degrees C). Press pie crust into the bottom and up the sides of an 11-inch tart pan.
Bake in the preheated oven until golden, 10 to 12 minutes. Remove and let cool.
Reduce the oven temperature to 325 degrees F (165 degrees C).
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels. Crumble.
Heat bacon grease in the same skillet over medium-high heat. Add onion and saute until translucent, about 5 minutes. Add apple and cook until just tender, 3 to 5 minutes. Add garlic and saute until fragrant, about 1 minute. Remove from heat and stir in bacon.
Whisk milk, sour cream, eggs, sea salt, and cinnamon together in a bowl.
Layer apple mixture, Cheddar cheese, and egg mixture in the prepared crust. Sprinkle sage over top.
Bake in the preheated oven until set, 30 to 40 minutes. Allow to cool before serving.