For the vegetables: Preheat the oven to 425 degrees F.
Trim the stems off the beets and lightly scrub with a vegetable brush - this will remove the tough outer layer. Quarter the beets and set aside. Trim the carrots (leaving a little of the stem on), and then cut in half crosswise. Add all the vegetables and the potatoes to a roasting pan along with the garlic head halves. Drizzle with the olive oil and season well with salt and pepper. Roast in the oven until golden brown and tender, about 45 minutes (the tip of a paring knife should pierce a potato and go through with little resistance).
For the spicy pepitas: In a large mixing bowl, combine the lemon juice, olive oil, chili powder, cumin and some salt and pepper. Add the pepitas and mix well so they are evenly coated. Toast in the oven until the pepitas are lightly browned, 7 to 10 minutes.
For the horseradish creme fraiche: Stir together the creme fraiche and horseradish and season with salt and pepper. Refrigerate until ready to serve.
To serve, place the roasted root vegetables on a large platter. Drizzle with the horseradish creme fraiche and garnish with spicy pepitas.