Preheat oven to 350 degrees.
In a large saucepan over medium heat, saute the sliced bacon for 3-5 minutes or until the bacon lightly golden brown and crispy. Transfer to a plate lined with paper towel and let cool. Reserve half of the bacon grease.
Reserve about 2 tablespoons of the bacon grease and discard the rest. Turn the heat back on to medium and add the onions. Cook until the onions are translucent and soft, then add the garlic and cook for another couple of minutes. Remove from the heat and set aside.
Wash and peel your potatoes. Slice into thin slices and add in overlapping layers to the bottom of a 9" x 13" baking dish. Season with salt and pepper, cover with 1½ cups of shredded cheddar cheese, and sprinkle on half of the cooked bacon.
In a large bowl, combine the milk, dry mustard, and paprika. Whisk well, then pour over the thinly sliced potatoes and cheese. Use a fork to lightly toss the potatoes so all the potatoes are covered in the milk mixture. Cover the layers of potatoes with the remaining cheese.
Cover tightly with aluminum foil and bake in your preheated oven for about 55 minutes. Remove the foil, then bake for another 10-15 minutes or until the cheese on top is golden brown and a fork inserted into the potatoes comes out easily.
Take the potatoes out of the oven and let them cool for 10 minutes before serving. Enjoy!