1. Preheat oven to 450°.
2. Heat oil in a large nonstick skillet over medium-high heat. Add onion to pan; cover and cook 3 minutes. Uncover and cook 10 minutes or until golden brown, stirring frequently. Top pizza crust with onion mixture; sprinkle with cheese. Layer pear and prosciutto over cheese; sprinkle with pepper. Place on middle rack of oven.
3. Bake at 450° for 12 minutes or until cheese melts. Sprinkle with walnuts.
4. Place arugula in a medium bowl just before serving. Drizzle vinegar over arugula; toss gently to coat. Top pizza with arugula mixture; cut into 8 wedges. Serve immediately.