Make a firm dough from butter, egg yolk, flour, salt and a little water, rest for about 1/2 hour. Roll out the puff pastry to a thickness of about 1-2 mm and leave to cool.
In a saucepan, melt the buter and sauté the finely diced onions. Add the carrots and kohlrabi and cook gently with a little water until soft, after about five minutes add the zucchini and after another two minutes the spinach, season with salt, pepper and a pinch of nutmeg, then leave to cool, drain and stir in the finely chopped Basils cheese.
Roll out the dough thinly and spread it on a round cake springform pan (28 cm), leaving a border of about 2-3 cm on the sides. Spread the vegetable-cheese mixture evenly on it and press it well until smooth. Bake the cake in the heated oven at 185 °C for about ten minutes, then pour the royal mixed with whipped cream, milk, eggs and seasoned with salt, pepper and nutmeg evenly over it and bake for another 20 minutes. Take out and rest a little. Cut out the puff pastry in the size of the springform pan, brush with egg and place on top of the vegetable cake. Bake at 185 °C for about ten minutes.
Serve the cake warm or cooled down.
Tip: Did you know that one bowl of spinach already covers the entire daily requirement of vitamin K and beta-carotene?