Roast the beets. I wrapped them unpeeled in aluminum foil and stuck them in the oven at 300 for about half an hour. Then I stuck a knife in them to make sure they were done. After they have cooled the skin comes right off and they can be sliced into rounds.
Blanch the beet greens. I put them in a large pot of boiling water for 1 minute, and then dunked them in a ice water bath and drained them. You can do the first two steps ahead of time and store them in the refrigerator.
When you are ready to make the gratin heat olive oil in a frying pan over medium heat. Add the garlic cloves and the greens and cook for 3 minutes.
Mix the eggs, milk, chives and scallions together, and put in a casserole dish.
Stir in the greens mixture and the sliced beets.
Add salt and pepper to taste, and then sprinkle the cheese over the top.
Bake 375 for 40 minutes, and cool 10 minutes before cutting.