1. Preheat the oven to 350 °F.
2. Line the bottom of a deep cake pan with parchment paper.
3. Put the potatoes in a large pot and cover with cold water. Bring to a boil; cook for about 10 minutes. Drain well and let cool slightly before slicing thinly.
4. Rinse the spinach, without draining; put it in a large pot. Cover and cook over low heat, stirring only once, until the spinach is just softened. Drain well and remove excess liquid. Chop finely.
5. Beat the eggs with the cream cheese and mustard seeds. Mix with the chopped spinach and herbs.
6. Arrange a layer of sliced potatoes in the cake pan, placing them in concentric circles. Deposit a few spoons of the cream cheese mixture and spread evenly. Season with salt and pepper. Repeat the same steps until both mixtures are used up.
7. Cover the pan with aluminum foil and place in a roasting pan.
8. Fill the roasting pan with enough boiling water to immerse half of the cake pan. Bake in the oven for 45 minutes. Serve warm or cold, on a bed of mixed lettuce.