Preheat the oven to 75 degrees. Separate or clean the leeks, onions, carrots and celery from the peel and finely dice or chop them. Rinse the tomatoes, discard the stem and cut off the tomatoes. Rinse the parsley, shake dry and chop. Put everything with the sea salt in hand mixer and make rather fine to puree.
Spread a gratin dish with parchment paper and put the quantity on it. Dry in the oven for one night (about 8 h). Grind the quantity finely in a hand mixer, passing it through a sieve in case it is necessary. The powder will keep for at least 10 weeks in a properly sealed container. For seasoning you need 1 tsp of powder for a mug of liquid.