Put flour, butter in flakes, 1 pinch of salt and 4 tablespoons of water in a bowl. Knead first with the dough hooks of the hand mixer, then with your hands to a smooth dough. Wrap in foil and chill for about 30 minutes.
Halve the avocados, remove the seeds, peel the flesh and cut the avocado into thin pieces. Sprinkle with lemon juice. Wash spinach, clean if necessary. Put avocado and spinach in a bowl and puree.
Grate the Parmesan finely and mix in loosely. Season to taste with salt and cayenne pepper. Chill the avocado mixture until used. Roast the pine nuts in a small pan without fat until golden brown, take them out and let them cool down.
Place the dough between two layers of foil and roll out rectangularly (approx. 22 x 30 cm). Place the dough in a rectangular tart tin (20 x 28 cm) with lift-off base, raising the edges. Line dough with baking paper, place dried peas on top.
Bake blind in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for 15-20 minutes. Remove baking paper and dry peas and bake for another 8 minutes at the same temperature.
In the meantime clean the mushrooms and cut them into slices. Peel and quarter sweet potatoes and cut into thin slices. Heat 2 tablespoons of oil in a large frying pan. Fry the mushrooms for 4-5 minutes, season with salt and pepper and remove from the pan.
Add 1 tablespoon of oil to the pan and fry the sweet potato slices for 1-2 minutes over a high heat while turning. Season with salt.
Remove the tart from the oven, carefully lift it out of the mould and place it on the sideboard. Spread the avocado cream loosely with a spoon on the tart base. Add mushrooms and sweet potato slices.
Sprinkle with pine nuts.