Step 1Heat the oven to 400 degrees. Trim and wash the beets and roast them, tightly covered, with a splash of water in a roasting pan until very tender, 50 to 60 minutes.
Step 2While the oven is on, place the walnuts on a baking sheet and toast them about 5 minutes. With a sharp paring knife, trim off the top and bottom of each orange. Pare off the rest of the peel, making sure to remove all of the pith. Slice the oranges into 1/4-inch rounds.
Step 3Make a vinaigrette by mixing together the vinegar, orange juice and the zest, and stirring in the olive oil and salt and pepper to taste.
Step 4When the beets are cool enough to handle, peel them and slice into rounds. Toss them gently with the vinaigrette and arrange the beets on a plate with the orange slices and Belgian endive leaves. Drizzle over any vinaigrette remaining in the bowl, and garnish with the toasted walnuts.