Using a food processor, pulse together the flour and salt until combined. Add butter and pulse until mixture becomes coarse meal, about 15 pulses.
In a small bowl, combine egg and water. Add the egg mixture to food processor and pulse until it starts to clump together. DO NOT pulse until a ball is formed. It should be crumbly with large clumps of dough.
Turn dough only a lightly floured surface and form into a ball. Flatten slightly to form a disk. Wrap in plastic wrap and put in fridge for at least 1 hour or up to 3 days.
If using mini tart shells, divide dough into 6 equal balls. On a lightly floured surface, roll each piece of dough into a circle slightly larger in diameter than the tart shell. Press dough into each tart shell, trimming the edges with a sharp knife. Cover each with plastic wrap and place in fridge for at least 30 minutes or until firm. NOTE - If using a 9 inch tart shell, just roll out to diameter slightly large and repeat steps above.
Preheat oven to 425°F. Place parchment paper over pie crust. Fill with pie weights or dried beans. Make sure the weights are evenly distributed around the pie dish. Place in oven and bake for 10 minutes. Remove parchment paper and pie weights and bake another 5 minutes. Let cool.
In a large skillet over medium heat, add bacon and cook to desired doneness. Drain on paper towel and reserve 2 tablespoon bacon grease. Add potatoes to skillet and cook until golden brown on all sides, about 7-10 minutes. Add onions and garlic and cook for another minute.
Evenly spread 1 ½ cups cheddar cheese across the bottom of the crust, dividing evenly. Add the potato mixture and then bacon, making sure it is even. In a large bowl, whisk together the eggs, milk and black pepper. Pour evenly over everything. Top with remaining ½ cup cheese.
Reduce heat to 350°F. Bake for 40-55 minutes or until the middle is just slightly jiggly. You can use a cake tester to test as well. You might need to cover crust with aluminum foil if it's starting to brown to quickly. Remove from oven and let sit for 15 minutes so it sets. (This is very important. If you don't let it sit for 15 minutes it will break up as you slice.)