Salad: Heat oven to 400 degrees F Trim beet stems. Put beets on a large piece of aluminum foil, drizzle with olive oil, and seal to make a tight package. Roast beets until easily pierced with a knife, about 1 hour. When cool, peel them (the skins should slide right off with a bit of pressure from your fingers). Cut into wedges or slices.
Dressing: Whisk vinegar, mustard, salt, and black pepper in a small bowl. Gradually whisk in oils to make a dressing. Stir in the shallot and thyme.
Toss greens, beets and walnuts with dressing. Divide the salad among 4 plates, add goat cheese and serve with toasted baguette slices.