Preheat the oven to 375 degrees F.
Place a large sheet of aluminum foil on a baking sheet. Place the beets on top, drizzle
with the olive oil and sprinkle with salt, then fold the long and short edges of the aluminum foil up to the middle and roll together, making a packet with the beets inside. Put in the oven and roast
until fork-tender, about 1 1/2 hours. (Can be done a day or two ahead of time.) Allow to cool, then slip the skins from the beets, trim the ends, and cut into 1/2-inch cubes. Set aside.
Quarter the endives lengthwise, then slice crosswise into 1/2-inch thick moons. Combine the beets and endive in a salad bowl and toss with the vinaigrette. Season with additional salt and pepper, if necessary. Serve immediately.