Pre-heat the oven to 425°F.
Place the potatoes in a large, shallow bowl or dish and season with salt and pepper; sprinkle with cornstarch.
Brush a large, 12-inch pizza pan liberally with olive oil. Arrange the potatoes around the pan, working all the way to the edges and overlapping the slices evenly to form the potato crust. Press the crust down to set, then sprinkle with 3-4 tablespoons stock. Brush liberally with olive oil. Bake for 15-20 minutes, or until the potatoes are cooked through.
In a skillet over medium heat, melt the butter. Add the onion and garlic and season with a little salt, pepper and the paprika. Cook to tender, 7-8 minutes. Add the corned beef and stock, then cover. Cook until crispy on the bottom.
Top the potato crust with a thin layer of sauerkraut, then with the corned beef and cheese. Bake or broil in the lower third of the oven to bubbly and brown.
In a small mixing bowl, mix together the sour cream, ketchup and relish. Transfer to a squeeze bottle and squeeze onto the top of the pizza, or dollop evenly around the pizza.