Stir the flour into the butter until smooth, then add the milk, stirring constantly.
Cook the sauce for about 1 minute until it thickens slightly, then add the Parmesan, garlic, salt, and pepper. Cook the sauce for another 1 minute until it thickens to a creamy consistency, then turn off the heat.
Cut the loaf of bread in half, then slice it through the middle to make 4 open-face pieces.
Put the bread on a sheet pan and spread each piece evenly with the white sauce.
Top the sauce with the onion and prosciutto.
Bake the pizzas for about 10-12 minutes, until the onions are soft and the prosciutto is crispy.
Remove the pizzas from the oven and top them off with the arugula.