Summer Squash Soup with Basil Pistou

Ingredients in this Recipe

1 medium leek, white and pale green parts only, thinly sliced and thoroughly rinsed
1 small clove garlic, minced
1 tablespoon unsalted butter
1/3 cup olive oil
2 1/4 pounds yellow summer squash, trimmed and cut into half moons
2 cups homemade or low sodium canned chicken stock
2 cups loosely packed fresh basil leaves, plus more for garnish
Salt and freshly ground pepper