Step 1In a medium, heavy-bottom saucepan, melt the butter with the oil over medium-high heat. Stir in the onion and cook, stirring occasionally, until the onion softens and takes on a golden shade, about 15 minutes. Stir in the garlic and cook just until aromatic, 1 to 2 minutes.
Step 2Stir in the white wine and cook, scraping any flavoring from the bottom of the pan, until the wine is almost completely absorbed. Sprinkle the flour over the onion mixture and stir to completely coat the onions.
Step 3Whisk in the milk and cream and bring the mixture to a bare simmer. Continue to cook, stirring constantly (and scraping the bottom of the pan to prevent flour from settling) until the mixture thickens to a sauce, about 3 minutes. Remove from heat, season with the salt and pepper and set aside.
Step 1Using a mandoline, slice the potatoes lengthwise as thinly as possible (they should be paper thin) into a large colander. Gently rinse the potato slices, washing off any excess starch. Shake the colander to drain the water and place the potatoes in a medium bowl.
Step 2Sprinkle the salt over the bowl and gently toss to coat the potatoes. Set aside for 15 minutes while the potatoes exude any excess moisture.
Step 3Grab a handful of potatoes and gently squeeze with your hands to drain the liquid. Place the drained potatoes in a large bowl. Repeat until all of the potato slices are drained.
Step 4Toss the drained potatoes with the olive oil, rosemary and Parmesan cheese.
Step 1Prepare the oven, setting the temperature as high as it will go (most ovens will reach 500- to 550-degrees). It's best to start heating the oven at least 45 minutes before you plan to start baking.
Step 2Place one pizza disk on a peel sprinkled with extra flour or cornmeal to prevent sticking. Spread one-fourth cup sauce over the pizza, then gently sprinkle over one-half of the potato topping in a single layer. Sprinkle over black pepper and sea salt to taste.
Step 3Slide the first pizza into the oven. The pizza will be done when the crust is puffed and rich golden-brown in color, the tips of the potato slices are crisp and golden and the sauce is bubbling gently, from 6 to 12 minutes depending on how your oven is set up (the hotter the oven, the shorter the time). Remove the first pizza and cool for a minute or two to set up for cleaner slicing, or set aside in a warm place until the second pizza is baked.
Step 4Allow the oven to reheat while you assemble the second pizza. Bake the second pizza and serve immediately.