Preheat oven to 425 degrees F. Whisk together vinegar, maple syrup, mustard, and fresh rosemary. Slowly drizzle in olive oil while whisking. Toss green beans in vinaigrette and spread out on a large baking sheet. (Tip: for easy cleanup line with a silicone baking mat, tin foil, or parchment paper.)
Top green beans with pieces of prosciutto and roast green beans for about 16-18 minutes or until starting to brown. Serve immediately