Heat oven to 400°F. Place beets on a piece of aluminum foil on a baking sheet and make a sack out of the foil, sealing tightly. Bake for 40 minutes. Remove from oven and let beets steam in sack for 10 minutes. Undo sack and let cool slightly.
Whisk the vinegar, mustard, agave nectar or honey, and garlic in a small bowl. Slowly whisk in the oils and season with salt and pepper.
After beets have cooled enough to handle, slip off their skins and slice into bite-sized pieces. Mix beets with half of vinaigrette. Mix spinach and arugula with the rest of the vinaigrette and top with beets, walnuts, crumbled goat cheese and serve. Serves 8.
Per Serving (5.5 oz-wt.): 180 calories (130 from fat), 14g total fat, 3.5g saturated fat, 6g protein, 9g total carbohydrate (3g dietary fiber, 4g sugar), 5mg cholesterol, 240mg sodium