Grease crock pot with oil or cooking spray. Layer 1/3 of onions, 1/3 of potatoes, and 1/3 of cheese. Repeat twice, ending with potatoes and cheese on top.
In large measuring cup, whisk together milk, cornstarch, yellow mustard, salt, and black pepper until completely smooth with no lumps.
Pour milk mixture evenly over all layers. Liquid should come three-quarters up the potatoes. Shake gently to help liquid settle.
Cover and cook on HIGH for 3-4 hours until potatoes are tender when pierced with fork and sauce is golden and bubbly.
Unplug slow cooker and allow potatoes to rest for 20 minutes before serving. This allows sauce to thicken further.
Notes
Use Yukon Gold or red potatoes for best results. Russets can become grainy in texture.
Mandoline slicer makes quick work of slicing potatoes uniformly for even cooking.
Yellow mustard adds flavor and color but doesn’t taste like mustard in final dish.
Oval slow cooker produces creamier results than round. Can use any 6-quart slow cooker.