Peel onion and finely chop. Blanch tomatoes for a few seconds, peel and cut into small cubes.
Rinse mussels. Peel garlic and chop finely.
Heat oil in a large skillet and sauté onion. Add rice and tomatoes and cook briefly, then pour in wine and simmer over a low flame.
Pan-fry chicken pieces on all sides in a pan with hot oil. Season with salt and pepper, and cook about 30 minutes, turning frequently.
In a covered pot, boil mussels in 400 ml (approximately 1 1/2 cups) of water about 7 minutes. Remove mussels, discarding any that remain closed. Pour the broth through a sieve into the rice and stir. Mix saffron with a little hot broth and add garlic. Season with salt and pepper and simmer about 20 minutes.
Slice chorizo and mix with the chicken pieces and rice. Top the paella with prawns, sprinkle with rosemary and bake in a preheated oven (200°C) (approximately 375°F) for about 10 min.