For the dough, put flour, 125 g butter in pieces, 1 pinch of salt and 4 tablespoons of cold water in a bowl. Knead first with the dough hooks of the mixer, then with your hands to a smooth dough. Wrap in foil and put in a cool place for about 30 minutes.
For the cream, clean and wash the spinach, drain well. Finely grate the parmesan. Cut avocados in half lengthwise and remove the seeds. Remove the flesh from the skin with a spoon and chop coarsely.
Put the lemon juice, spinach and parmesan into a tall mixing bowl and puree with a hand blender. Season to taste with salt and cayenne pepper. Cover the avocado cream and chill.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Roll out dough between two layers of foil in a rectangular shape (approx. 24 x 32 cm). Place the dough in a greased rectangular tart tin with a raised base (20 x 28 cm), raising the edges.
Line the dough with baking parchment, spread legumes on top. Bake in a hot oven for 15-20 minutes. Remove baking parchment and legumes. Bake at the same temperature for another 8 minutes.
In the meantime clean the mushrooms, wash them if necessary and cut them into thin slices. Peel, wash and quarter sweet potatoes and slice or cut them into thin slices. Roast pine nuts in a pan without fat until golden brown, take them out.
Heat 2 tablespoons of oil in a frying pan. Sauté the mushrooms in the oil for 4-5 minutes while turning. Season with salt and pepper and remove. Heat 2 tablespoons of oil in the frying fat one after the other. Fry the sweet potato slices for 1-2 minutes, turning them in portions.
Season with salt.
Remove the tart from the oven, let it cool down briefly, then carefully lift it out of the mould and place it on a plate. Spread the avocado cream loosely with a spoon. Add mushrooms and sweet potatoes.
Sprinkle with pine nuts and serve immediately.