Preheat oven to 400°.
To prepare salad, combine first 4 ingredients, tossing well. Place potato mixture on a jelly-roll pan lined with parchment paper. Bake at 400° for 40 minutes, stirring occasionally. Remove from oven; cool. Discard garlic. Combine potato mixture, orange sections, onion, pine nuts, and spinach in a large bowl.
To prepare dressing, combine orange juice and remaining ingredients in a small bowl, stirring well with a whisk. Drizzle dressing over salad; toss gently to coat.