Remove about half of the bacon grease so there are a couple of tablspoons left in the pan. (I like to use a paper towel to soak some of it up. You can let that paper towel sit on a plate until it is cool enough to throw away.)
Add the onions to the bacon grease. If you removed too much of the bacon grease, you can melt a couple of tablespoons of butter in the saucepan. Saute the oni0ns on medium heat until the onions are translucent and soft. Add the garlic and saute for a couple more minutes until the garlic is also soft. (It's okay if the onion turns slightly golden. Use a spatula to scrape the bottom of the pan to remove any of the flavorful bacon bits off the bottom of the pan.) Remove from heat and set aside.
Wash and peel your potatoes. Slice the potatoes into thin slices and add to a baking dish. A 9 by 13" casserole dish works great. Season with salt and pepper. Cover with 1 1/2 cups of shredded cheddar cheese --or your favorite cheese, and sprinkle on half of the cooked bacon.
In a large bowl, to make the creamy cheese sauce, combine the milk, dry mustard, and paprika. Whisk. 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, the scalloped potatoes with aluminum foil and bake at 350 for 55 minutes. The Take the foil off. The edges of the baking dish should be bubbly. Bake for another 10-15 minutes until the cheese on top is golden brown.
Check the doneness of the potatoes by checking with a fork. (Your fork should easily be inserted and the potato should break apart easily and have no resistance when pulling the fork out. If potatoes aren't quite done, bake for another 10-15 minutes or until the potatoes are fully cooked. Cover with foil again if necessary so your cheese isn't overdone.
Take the potatoes out of the oven and let them cool for 10 minutes before serving. Enjoy!