Soak porcini mushrooms in warm water for about 15 minutes. Peel onions and garlic and dice finely. Sort the spinach, wash and drain. Wash and halve the tomatoes. Wash lemon hot, dry and grate the peel finely. Cut the lemon in half and squeeze one half.
Pour the porcini through a sieve, collect the soaking water and fill it up. Squeeze the mushrooms and chop them finely. For the polenta 1⁄2 boil up l water, milk and 1 level teaspoon salt. Stir in the polenta and bring to the boil. Stir in porcini. Let it swell for 10-15 minutes at low heat, stirring from time to time. Stir in butter. Season to taste with pepper, lemon juice and a little salt.
Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Heat oil in a large pot. Sauté onions and garlic in it. Add the tomatoes and spinach, cover and allow to collapse or heat for 3-4 minutes. Season with salt, pepper and lemon peel. Distribute in a casserole dish. Spread the polenta on the spinach. Beat the eggs one after the other and put them on the polenta. Season with salt. Bake in a hot oven for 25-30 minutes until the eggs have set.