Preheat oven to 425 degrees. Lightly spray or rub each beet in olive oil. Wrap each beet tightly in tin foil or parchment paper. Roast the beets for 45-60 minutes, or until they can be easily pierced with a knife. Allow the beets to cool at room temperature for about 20-30 minutes. Then, run them under cold water, slipping the skins off with your fingers. Chop the beets and set them aside.
Place the beets, chickpeas, and garlic in a food processor fitted with the S blade. Pulse to combine. Add the salt, tahini, lemon, and water. Process the mixture for a full 3-4 minutes, or until the hummus is super creamy, stopping to scrape down the bowl now and then. Alternately, you can add all of the hummus ingredients to a high-speed blender and process till smooth.
Pulse in the rosemary (optional) and freshly ground black pepper to taste. Serve the hummus with crackers, crudités, pita wedges, or however you like!