For the dough cut butter into pieces. Knead butter, flour, egg yolks, salt and 2-3 tablespoons of ice water with the dough hooks of the hand mixer, then work into a smooth dough with your hands. Wrap the dough in foil and chill for about 30 minutes. Wash, clean and slice the zucchini. Peel onions and cut into slices.
Drain the mussels well in a sieve. Roll out the dough on a floured work surface to a circle (30 cm Ø). Put the dough into a greased tart tin (26 cm Ø). Press the rim lightly. Spread the zucchini, mussels and onions evenly in the tin. Grate cheese. Wash parsley, shake dry. Pluck the leaves from the stems and chop them, except for a little bit for garnishing. Mix milk, eggs, cheese and parsley. Season with salt and pepper and spread over the tart.
Wash parsley, shake dry. Pluck the leaves from the stems and chop them, except for a little bit for garnishing. Mix milk, eggs, cheese and parsley. Season with salt and pepper and spread over the tart. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 45-60 minutes. Let the finished tart rest in the pan for about 5 minutes, carefully remove from the pan and cut into pieces. Garnish with parsley
Preparation time approx. 1 hour 10 minutes. waiting time approx. 30 minutes