Preheat the slow cooker to Low. Place the tongue, onion, carrot, garlic, bay leaf, thyme, parsley, and peppercorns into the slow cooker insert. Add water to cover. Cover and cook on Low for four to five hours. Remove the tongue from the liquid and set aside to cool. Discard the cooking liquid.

When the tongue is cool, peel off the outer skin and trim away the fatty underside.

Place a large sauté pan over medium heat. Lightly coat the bottom of the pan with olive oil. Brown the tongue on all sides. Refrigerate the tongue until it is completely cool.

While the tongue is cooking, preheat the oven to 350 degrees Fahrenheit. Coat the beets lightly with oil, wrap in aluminum foil, and roast for one hour or until a knife pierces them easily. Remove from the oven and let cool. When cool, slip off the skins and slice the beets into about half-inch rounds.

Place the frisée on a platter. Arrange the beets, fennel, celery, carrots, and onion on the frisée. Thinly slice the tongue and arrange the slices on top of the vegetables.

Dollop some of the aïoli (recipe below) on top of the meat. Pass additional aïoli on the side.

Place the garlic, egg yolk, horseradish, water, and lemon juice in a large nonreactive bowl. Whisk together until smooth. Slowly drizzle in the oil while continually whisking. The mixture should resemble a loose mayonnaise. Add salt and pepper to taste.

You can also make this delicious aïoli in the food processor. Lightly pulse the garlic, egg yolk, horseradish, water, and lemon juice until combined. With the motor running, drizzle the oil in a steady stream until the mixture becomes thick and creamy. Add salt and pepper to taste. Serve or store.