Spread the diced sweet potatoes and sliced onion on a pan. Drizzle with olive oil, add a pinch of salt and pepper to taste. Arrange in a single layer and roast for about 35-40 min or until cooked and golden.
Boil farro in three cups of water with a pinch of salt for 15-20 minutes. When the farro is cooked, drain any excess liquid and set aside.
Mix all ingredients in a bowl, taste for seasoning, adjust if necessary and set aside.
To assemble the salad set a bed of greens on a serving dish, then add a layer of farro or the grain you are using, then the roasted vegetables add the hazelnuts and finish with the "honey-mustard" dressing.