1. Peel the sweet potato and cut out 4 even slices, about 4 mm thick. Set the remaining sweet potato aside for another use. Toast the slices in the toaster until the potato meat softens and the surface darkens and bubbles a little.
2. In the meantime, loosen the nomad's cheese with a fork, scrape into a bowl and mix with the mustard. Fill a large pot with water up to about 5 cm, add the vinegar and bring to a boil. Meanwhile, wash the spinach and spin dry. Peel the onion and cut into a fine dice. Heat the butter in a pan and sauté the onion over medium heat until translucent. Add the spinach and stir, cooking until the spinach wilts. Season with nutmeg, salt and pepper.
3. When the water in the pot begins to steam and small bubbles form on the bottom, reduce the heat. Using a fork, stir the water in a circular motion several times to create a whirlpool. Pour 1 egg into a small glass, hold the glass close to the surface of the water and let the egg slide into the whirlpool. Let the egg continue to stir in the boiling water for about 4 minutes. If it sinks too low, use a spoon to prevent it from attaching to the bottom of the pot by gently moving it. Carefully lift out the egg with a slotted spoon and place it on a plate. Do the same with the remaining eggs.
4. If necessary, toast the sweet potato slices in the toaster briefly to warm. Spread the slices generously with the Tulum and mustard mixture. Add some spinach on top, place 1 poached egg on each slice, sprinkle with fleur de sel and pepper and serve garnished with cress.