Cut the onion into rings, water the anchovies and mussels. preheat.
Roll out the puff pastry on a floured surface and place in a springform pan with a diameter of approx. 26 cm. Pull up an edge of approx. 2 cm and prick the dough lightly over the entire surface with a fork. Mix the tomato paste well with 1 tablespoon of oil. Brush the dough with the remaining oil.
Skin and slice the tomatoes. Now brush the pastry base with the tomato paste and cover with the tomatoes. Chop the tuna and spread on the tomatoes with the onions, anchovies, crabs and mussels. Sprinkle the grated Gouda over the pizza.
Preheat the oven to around 220 ° C. Put the pizza on the middle rack in the oven and bake until the cheese is nice and golden brown. Serve hot.