Boil the peas in salted water. Once boiled, drain and blend together with the butter at room temperature. Add salt if necessary.
Clean the sea bass, season with salt and pepper, drizzle with olive oil, and cook in a preheated oven at 180 ֯C (356 °F) for 25-30 minutes.
Leek ash can be made in advance and kept in a sealed container.
Clean the leeks and cut down the middle. Open up the layers and bake in the oven at 180° C (356° F) until completely blackened and crispy. Remove them from the oven, leave to cool, and grind them into a powder.
Serve the sea bass together with the mashed peas and a sprinkle of leek ash on top. Enjoy!