In a large saucepan, heat the olive oil over medium-high heat. Add the 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 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, make the Pistou: Put the garlic and tomatoes in a blender or food processor with about 3 tablespoons of the olive oil. Begin processing. With the machine running, add the basil leaves, and then pour in enough extra-virgin olive oil to make a smooth, thick, but fluid paste. Transfer about 2/3 of the paste into a serving bowl to pass alongside the soup.
Make the Goat Cheese Croutons: Preheat the oven to 375 degrees F.
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 pistou into the saucepan, ladling some of the hot broth into the blender or processor bowl to swirl and rinse any pesto clinging inside into the pan. 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 alongside for each person to add, to taste.