Place sweet potatoes and carrots in a steamer basket. Cover and steam vegetables over simmering water for 10 minutes or until tender, adding the onion the last 1 minute of steaming. Cool.
Meanwhile, for dressing, in a screw-top jar with a tight-fitting lid, combine olive oil, vinegar, mustard, salt, and pepper. Cover and shake well to combine.
Place cooled potatoes, carrots, and onion in a salad bowl. Gently fold in snipped fresh thyme. Top with ham and cheese. Drizzle with dressing; top with thyme sprig.