Position a rack in the center of the oven and heat the oven to 450°F. Line a rimmed baking sheet with foil. On the baking sheet, toss the onion, sweet potato, 2 Tbs. of the oil, 1 tsp. of the rosemary, 1/2 tsp. salt, and 1/4 tsp. pepper and spread in a single layer. Roast until the vegetables start to become tender, about 15 minutes.
In a small bowl, toss the ham with the maple syrup. Push the vegetables on the baking sheet aside to make room for the ham and bake until the ham and onions are browned in places, about 10 minutes.
Meanwhile, in a small bowl, whisk the remaining 6 Tbs. oil with the vinegar, the remaining 1/4 tsp. rosemary, and 1/4 tsp. each salt and pepper. In a large bowl, toss the mesclun with 1/4 cup of the vinaigrette. Season to taste with salt and pepper.
Divide the mesclun among 4 plates. Top with the roasted vegetables and ham. Sprinkle each salad with some of the goat cheese. Drizzle with the remaining vinaigrette and serve.
This hearty salad needs only a loaf of crusty bread to make a complete meal. For dessert, try couscous pudding (like rice pudding, only quicker), studded with brandied prunes or dried apricots.