Clean the mussels thoroughly (Scraping the remains stuck on the shells, removing whiskers…). Chop the onion and leek into very small squares; the celery, tomato and carrot in pieces.
Add a couple of tablespoons of virgin olive oil in a pan. When hot, add the onion, carrot, garlic and leek, add salt and pepper, cover and lower the heat, let cook until the onion is transparent.
Add the tomato and celery, put back on the heat until it begins to bubble, lower the heat, cover and let it finish cooking until the tomato is in place. Put the mixture through a sieve so we have a fine sauce. Add salt to taste.
Put the mussels in a hot pan, cover them with the tomato sauce and cover them. Let them cook and remove the ones that have opened, placing them in a dish to avoid cooking too much.
When all the mussels are cooked, place the tomato sauce on top and add the chopped basil on top. Serve.