Preparation (15 minutes): Start by slicing the potatoes thinly using a mandoline slicer or a sharp knife. Rinse them in cold water to remove excess starch and set aside.
Prepare the Creamy Sauce (10 minutes): In a large saucepan, combine the heavy cream, whole milk, minced garlic, onion powder, dried thyme, dried rosemary, salt, and black pepper. Heat the mixture over medium heat, stirring occasionally until it simmers. Remove it from the heat and set aside.
Layer the Potatoes (10 minutes): In each sterilized canning jar, layer the sliced potatoes, slightly overlapping them to create a beautiful presentation. Optionally, add a layer of shredded cheddar cheese for extra creaminess.
Pour the Creamy Sauce (5 minutes): Carefully pour the creamy sauce over the layered potatoes, filling the jars almost to the top. Leave about 1/2 inch of headspace to allow for expansion during canning.
Canning (20 minutes): Wipe the jar rims clean, place the lids and bands, and tighten them securely. Process the jars in a water bath canner or a pressure canner according to the manufacturer's instructions. Processing time will vary based on your altitude and canner type.
Cooling and Storage (1 hour): After processing, remove the jars using a jar lifter and let them cool on a towel for 12-24 hours. Check the lids to ensure proper sealing; they should be concave and not flex when pressed. Store the sealed jars in a cool, dark place for up to one year.