In a medium saucepan, heat orange juice over medium-high heat, whisking often, until it is reduced by 1/4.
Add to a blender, along with the vinegar, onion, sugar, wine, salt and pepper, and process until smooth. Then, while blending on a low speed, remove cap and slowly drizzle in the olive oil to emulsify (thicken) the dressing. Chill until ready to use.
Melt butter in a non-stick skillet over medium heat. Saute pears and nuts in butter for 3 minutes. Add sugar and water and cook, stirring constantly, until golden brown and caramelized. Remove from heat and set aside to cool.
In a large bowl, combine prosciutto, lettuce and the pear and walnut mixture. Add vinaigrette and toss to coat. Serve on a large Italian platter. Enjoy!