Place the beets in a saucepan and add water to cover with salt. Bring to a boil until the beets are tender, about 30 minutes, depending on the size or age of the beets. Drain and let cool. Remove the skins and slice the beets. Trim off the bottom of the endives and cut them into 1 1/2 inch strips. Drop the pieces into cold water. Drain and pat dry. Peel and slice the onion. Combine the mustard, vinegar, salt and pepper in a salad bowl. Add the oil and blend well with a wire whisk. Add the beets, endive, onion and parsley. Toss well and serve.