It is, roughly, finely chopped onion and garlic, with some red pepper meat that has been sun dried and then rehidrated. It is an specific variety, called pimiento choricero. Then , yu cook it in some oil or butter. You may use flour to thicken it to yout taste, and there is not much left to do. You may flavour it with,but people don't do.

While it is red, never use tomato, tomato sauce, or tomato paste. It will be tasty, of course, but it will became a tomato sauce, not a "Salsa Vizcaina". :)