This is the perfect combination for spring when asparagus and spring onions are at their sweetest. Cut the asparagus spears on the diagonal if they are extra thick. Top the baked pie with dressed arugula if you like.
Heat the oven to 450°F and set an oven rack in the middle position.
Grease a large, rimmed baking pan (18 x13 inches) with olive oil. On a lightly floured work surface, roll the dough out into a rectangle approximately the same size as the pan. Transfer the dough to the pan and, using your fingers, press the dough to the edges and up the sides; patch any holes that form.
Brush the dough with 2 Tbs. of the oil and then top with the asparagus, the white parts of the onions (or scallions), and the mozzarella. Drizzle with the remaining 1 Tbs. olive oil and sprinkle generously with salt and pepper.
Bake the pizza until the dough browns on the bottom and sides and the cheese melts, 15 to 20 minutes.
Lay the slices of prosciutto across the top. Sprinkle on the scallion greens and the sun-dried tomatoes, and slice the pizza into square pieces. Serve immediately.