Slice the onion and place in the bottom of a large saucepan. Place the roast rind-side down on top of the onion. Pour boiling water over it. Add the caraway seeds, mugwort, carrot, and celeriac. Season with pepper only, do not add salt yet. Cover the pot and simmer on the lowest heat for 1 hour.
Preheat the oven to 200°C (400°F) with top and bottom heat. Remove the roast from the pot. Using a sharp knife, score the rind in a diamond pattern. Season all sides generously with salt and pepper. Strain the cooking liquid and lightly salt it.
Place the roast in a roasting pan with the rind facing up. Pour some of the cooking liquid into the pan. Roast for 2 hours, basting with some of the cooking liquid every 20 minutes. 5 minutes before the end of roasting, brush the rind with ice-cold salted water. The roast is done when the internal temperature reaches 63°C (145°F).
Remove the roast from the oven. Scrape the browned bits from the bottom of the roasting pan into the cooking liquid. Season the sauce to taste. If it is too strong, dilute with some water; if it is too thin, simmer for 3–5 minutes to reduce and concentrate the flavor.