In a medium saucepan over medium heat, whisk all the ingredients through the rice vinegar, whisking constantly until bubbly and thick.Slice the potatoes paper thin. You can use a mandolin for this, or your food processor with the slicing attachment (my favorite small appliance!) You could also do this by hand, but getting them very thin and even is much more difficult! Mix the potatoes and onions in a greased 11x17pan.Pour the sauce over the potatoes. I have used different sized pans for this, and I adjust the time for cooking up or down as necessary. Most important is to have fork-tender potatoes at the end!Sprinkle with the paprika.Bake at 350 degrees for 1-1/2 hours until bubbly and golden.