Heat some of the stock in a pot and add the mussels.
Parboil them just until they open up.Strain them and mix the liquid back into the fish stock.
In a paella pan or deep skillet heat olive oil and add the Shrimp.Cook until pink on both sides.
Keep aside. In the same oil add the Chorizo sausage and brown them on both sides.
Keep aside.Add the onions and garlic into the skillet along with tomato paste and can of tomatoes.
Fry for about 2 minutes and add the rice and saffron.Add the fish stock and bring it to a boil.
Once it starts boiling reduce the heat, cover the skillet with a tight fitting lid and cook for 15 minutes or until all the stock is absorbed.
Open the lid and arrange the Shrimp, sausage, mussels and peas and put the lid back on.
Let it cook for 5 more minutes.
You can serve the Paella as it is or transfer in a tray.Garnish with fresh parsley!