Remove dough from the refrigerator 1 hour before using to allow the dough to come to room temperature.
To make béchamel sauce: In a 2-quart saucepan over low heat, melt butter. Stir in flour and cook for 3 minutes. Gradually add milk, stirring constantly. Increase heat to medium. Cook for about 5 minutes or until the sauce thickens. Season with salt and pepper. Stir in parmesan. Reserve.
Preheat oven to 450 F.
Lightly oil a 15" pizza pan and dust with corn meal. On a lightly floured board, roll out one round of pizza dough into a 15" circle. Place dough in pan and crimp a one inch rim around rim. Brush rim with 2 tablespoons olive oil and sprinkle with sea salt. Top with béchamel sauce.
For the topping, lightly oil a baking sheet. Place sliced potatoes in a medium bowl and season with salt and pepper. Drizzle with 1 tablespoon olive oil, toss to coat evenly. Place on prepared baking sheet and roast until browned, about 10 minutes.
Place spinach, parsley, basil, garlic, walnuts and parmesan in the large bowl of a food processor. Pulse to combine and form into a smooth paste. Spread thickly on top of béchamel sauce. Top with roasted potatoes. Bake in 450 F oven until dough is lightly browned, about 15 minutes. Serve hot.
Makes one 15" pizza.