Preheat oven to 425°F. Wrap beets in foil. Place in shallow baking pan. Roast 1 hour or until beets are tender when pierced with small knife. Cool until able to handle. Peel then coarsely chop beets.
Drain beans, reserving 1/4 cup of the liquid. Place beets, beans, tahini, lemon juice and seasonings in food processor; cover. Process until smooth, scraping down sides as necessary. Add reserved liquid; process until smooth. Spoon dip into small bowl. Cover. Refrigerate until ready to serve.
To serve, drizzle dip with olive oil and garnish with chopped toasted pistachios or pepitas, if desired. Serve with warm pita wedges or pita chips.