Heat oven to 400 degrees. Wash beets and cut the stalks to 1/2 inch. Reserve beet greens. Put beets on a sheet of aluminum foil and form a pouch, sealing tightly. Place on a baking sheet and bake for 35 minutes.
Remove from oven and allow beets to steam in pouch 10 minutes longer. Take beets out of pouch; place in refrigerator. When cool, slip beets out of their skins and slice. Set aside.
To make vinaigrette, whisk together mustard, vinegar, and sugar in a small bowl. Slowly whisk in the oil, and season with salt and pepper.
Toss beets with the vinaigrette and serve on a bed of arugula.
For a delicious alternative to the usual leafy vegetables, place washed beet greens in pot with a little olive oil and some minced garlic. Cover and turn up heat to medium. Cook just until wilted.