Clean and wash the mussels thoroughly. Open them and sauté them in a pot, over medium heat, with oil and chopped onion. Let it brown and add the rice to toast. Add the wine and let it evaporate then start cooking the rice by gradually adding the mussel water and stock. Add the pesto and then add the mussels a few minutes before the rice is cooked. Cream with butter and extra virgin olive oil. Plate and add a pinch of black pepper.