Pistou is quite simple to prepare and it can be made in either a food processor or a mortar and pestle. The latter will give you a better texture, but either method will work just fine. In addition to Soupe au Pistou, the pistou can also be used as a sauce for pasta, chicken, fish and vegetables.
3 or 4 cloves of garlic, (sliced, adjust amount according to your taste)
pinch of sea salt
1 1/2 cups of basil leaves, torn
1 1/2 cups flat-leaf Italian parsley leaves, chopped
1 1/2 cups celery leaves, chopped (you can also make pistou with just basil, or a mixture of equal amounts of basil and parsley)
1 fresh tomato, cored and chopped
extra-virin olive oil, about 1/2 cup, but you may need more
1/4 cup of grated Parmigiano-Reggiano or an aged Gruyère or a mixture of equal amounts of Parmigiano-Reggiano and Pecorino Romano, or whatever you prefer
If you are using a mortar and pestle, add the garlic and a pinch of sea salt to the mortar and use the pestle to make a paste. Gradually add the basil, parsley, and celery leaves, grinding the leaves with the pestle against the sides of the mortar, until a soft paste is formed. Add the chopped tomatoes and combine with the herbs. Slowly add 1/2 cup of olive oil and mix with the pestle until all of the oil has been incorporated and you have a smooth mixture. Add additional olive oil if needed. Add the cheese and mix with the pestle. Taste, adjust the seasoning, as well as the amount of oil and cheese. Stir before serving
If you are using a food processor, add the garlic, tomatoes, a few tablespoons of olive oil and a pinch of sea salt to the bowl. Start to process the tomatoes and garlic. While the machine is running, add the basil, parsley and celery leaves. Then, while the machine is still running, slowly add 1/2 cup of olive oil, and process until you have a smooth mixture. Add additional olive oil if needed. Place in a bowl and mix in the cheese. Taste, adjust the seasoning, as well as the amount of oil and cheese. Stir before serving.
If you are not ready to use the pistou in the soup, transfer to an air-tight container, drizzle some additional oil over the top, and refrigerate for up to 1 day or freeze for up to 6 months. (Also if you are going to refrigerate the soup overnight, the pistou should not be added to the soup until just before it is served. If the pistou is mixed with the soup, refrigerated and then reheated, it will probably lose its vibrant green color.) Bring to room temperature and stir again before serving.