To make the tomato sauce, heat the olive oil in a deep pan. Add the onions, garlic and pepper and saute for 5 min. Add the tomatoes, tomato puree, sugar, basil and bay leaf. Season with salt and black pepper. Put a lid on and saute for 30 min.
Roll out the pizza dough on a floured surface and transfer it to a baking tray. Smear on the tomato paste. Cover 1/4 of the circle with mussels.
On the 2nd quarter, place the chopped artichoke, cover the 3rd quarter with slices of tomatoes and mozzarella and the last - with sliced mushrooms. Sprinkle with olives and capers, season with salt, black pepper and a little olive oil.
Bake the pizza in a preheated 428°F (220 °C) oven for 20-25 min.