Bring 2 liters of salted water to a boil in a large saucepan over high heat. Peel 1 onion and stud it with 2 cloves. Remove roots from 300 g celery, brush it, and cut in half. Add the 800 g beef, studded onion, celery, and 2 bay leaves to the boiling water; add 2 teaspoons sea salt. Optionally add 1 chopped carrot and 1 small leek.
Reduce heat to low, cover the pot, and simmer gently for 1 hour 30 minutes—keep the broth at a gentle simmer, not a rolling boil, so the meat stays tender.
Remove the meat from the broth and cover it. Let rest for 10 minutes so the juices settle, then cut into slices.
Peel 100 g fresh horseradish and rub it in a bowl with 3 tablespoons of milk (if possible, freeze the horseradish first to avoid tears). Remove 125 ml of meat stock from the saucepan and let it cool to room temperature.
Heat 1 tablespoon oil in a small saucepan over medium heat. Add 1 tablespoon flour and stir for 1-2 minutes until light yellow.
Pour in the remaining milk, whisking vigorously to prevent lumps. Simmer for 2-3 minutes, stirring occasionally, until creamy.
Stir in the cooled meat stock and horseradish mixture. Season with 1 pinch of sugar, salt, and pepper to taste.
Arrange the sliced meat on a serving platter, pour the horseradish sauce over it, and serve with beetroot salad.