The Provençal dish Soupe au Pistou is a summer soup that includes summer vegetables and a Pistou sauce. Pistou is a cold sauce made from cloves of garlic, fresh basil, and olive oil. Pistou can be incorporated into the soup just before serving or offered at the table to be added after the soup is served, or both.
Trim the dark green parts of the Leeks and discard. Wash the light green and white parts thoroughly and cut into 1/2-inch dice.
In a large saucepan, heat the olive oil over medium-high heat. Add the (summer) leeks and saute just until they start to turn translucent, about 3 minutes.
Add the garlic and saute about 1 minute more. Add the carrots and celery and continue sauteing until the vegetables deepen in color but have not yet begun to brown, 3 to 4 minutes more. Pour the stock (or water to make the dish vegetarian) into the pan, bring it to a boil, and reduce the heat to maintain a simmer. Sprinkle in a generous pinch of salt.
Tie the peppercorns, thyme, and parsley in a square of cheesecloth, securing it with kitchen string. Add this bouquet garni to the pan. Stir in the beans, zucchini, summer squash, and tomato. Continue simmering until the vegetables are tender, about 30 minutes.
Meanwhile, using ingredients marked (P) make the Pistou: Put the garlic and tomatoes in a blender or food processor with about 3 tablespoons of the olive oil and a generous pinch of salt. Blend into a paste. Add the basil a handful at a time and continuously blend making a smooth, thick fluid paste. Transfer about 2/3 of the paste into a serving bowl individuals to add to their soup. Keep 1/3in the blender to add to the soup in step 3. below.
Preheat the oven to 375 degrees F.
Using ingredients marked (C), brush the bread slices with olive oil and arrange them on a baking sheet.
Bake until golden, 12 to 15 minutes. Remove them from the oven and turn on the broiler.
Spread 1 side of each crouton with goat cheese and put it back on the baking sheet. Broil until the cheese is warm and slightly bubbly, about 3 minutes. Set aside.
When the soup is ready, stir the remaining 1/3 of the pistou into the saucepan, ladling some of the hot broth into the blender bowl to swirl and rinse any pistou clinging inside into the bowl. Remove and discard the bouquet garni. Taste the soup and adjust the seasoning with a little more salt.
Ladle the soup into individual heated serving bowls. Float a crouton in the center of each bowl. With a spoon, drizzle a little more pistou over the crouton and soup in each bowl. Serve immediately, passing more pistou for each person to add, to taste.