adapted from William Werner
For oven roast: Preheat oven to 375°F. Cut off beet greens, leaving 1-inch stems; leave root intact. Scrub clean with water and place in a glass baking dish. Add water to about ¼ inch, cover tightly with foil, and bake for about 45 minutes, until beets are fork-tender. When cool enough to handle, peel then slice into rounds or cut pole-topole into 8 wedges (or cut some of each).
For sous vide: Trim off the roots and greens, wash and peel the beets. Place the beets in a sous vide bag (I kept the yellow beets separate from the red by using two bags) and drizzle in a tablespoon of olive oil and a teaspoon of wine vinegar. To make a sous vide bag, use FoodSaver vacuum bags, Zip-lock freezer bags, or silicone Stasher bags—just keep in mind that the bag’s color will never be the same after you cook red beets in it. Cook to 185°, which takes about 2 hours for 2-inch beets. Remove from water bath and let cool. Refrigerate until you are ready to make the salad.
Preheat oven to 350°. Put hazelnuts on a rimmed sheet pan or pie plate and bake until they’re fragrant and their skins darken and begin to blister, about 5–10 minutes or longer, depending on the particular nut. Transfer to a clean “working” kitchen towel (the skins will stain) and rub with your hands to remove some of the skins. Set aside to cool completely. (You may not use all of these; keep the remaining hazelnuts in a jar at room temperature and enjoy them throughout the week.)
Stir together in a small jar.
Arrange on a platter or plate with fennel bulb slices on the bottom, followed by beets, oranges, hazelnuts, and fennel fronds. Drizzle with crème fraîche dressing and sprinkle with flake salt.