Heat 2 tablespoons of the oil in a large saucepan over medium.
Add garlic and cook, stirring, until fragrant (1 minute).
Stir in beans and tomatoes and season with salt and pepper. Cook, stirring occastionally, until tomatoes soften (about 3 minutes).
Stir in clam juice and pimentón and bring to a boil.
Coat black cod with the remaining 1 tablespoon of oil and season with salt and pepper. Place fish in pan, cover and simmer over medium heat for 5 minutes.
Add cockles, cover and cook until cockles open and fish is cooked through (about 3 minutes longer).
Transfer fish to 2 shallow bowls.
To pan, add butter, lemon juice and parsley and mix well.