Preheat the oven to 275 degrees F. Place the beets in a bowl and toss with 1 tablespoon of the olive oil. Set the beets in a parchment paper-lined baking dish. Roast for 1-1/2 hours, or until the beets are tender. Cool the beets until safe enough to handle, and then slip or carefully peel off the skins. Slice the beets into thin rounds. Divide and arrange the beet slices on the outer edge of 4 salad plates; sprinkle them with salt and pepper. Place the remaining olive oil, orange juice, balsamic vinegar and mustard in a medium bowl and whisk to combine; season with salt and pepper. Add the spinach and toss to coat. Mound the spinach in the centre of the salad plates and top with the goat cheese and orange.