Heat oven to 400 degrees F. Stir together vinegar, maple syrup and mustard. Slowly whisk in 2 tablespoons of the olive oil; set aside.
Place sweet potatoes, onion and ham in a large bowl. Drizzle with remaining 1 tablespoon olive oil; stir until coated. Spread on a rimmed baking sheet; sprinkle with thyme, salt and pepper. Roast at 400 degrees for 45 minutes, stirring halfway through.
Remove from oven and place in a large bowl. Stir in spinach and walnuts. Drizzle with maple vinaigrette and serve immediately.