Place 1 cup of water in the cooker and bring to a boil.
Add the mussels and cook until they open.
Discard the top shells and reserve the mussels on the bottom shells on a warm platter covered with foil.
Wash and dry the cooker thoroughly. Heat the oil until very hot and sauté the fish and the shrimp until they are just cooked. Move to the platter and cover tightly with foil.
Add the onion and garlic to the cooker and sauté until the onion is wilted. Stir in the pimiento, parsley, paprika, and then the tomato. Cook for 3 minutes.
Stir in the rice and coat well. Pour in the stock then add the peas and the saffron and bring to a boil.
Close the lid, bring to pressure and cook for 3 minutes on HIGH pressure or 6 minutes on LOW pressure.
After the time is up, remove the pressure cooker from the flame and let the pressure release naturally.
After the pressure has dropped, let sit, closed, for an additional 4 minutes. Remove the lid and stir in the reserved mussels, fish, and shrimp.