For squash soup:

3/4 stick (6 tablespoons) unsalted butter, cut into pieces

1 medium onion, halved lengthwise and thinly sliced crosswise

1/2 teaspoon salt

3-4 yellow summer squash, halved and thinly sliced

2 carrots, thinly sliced

2-3 new potatoes  halved

4 cups chicken or vegetable stock

for pistou:

1 bunch fresh flat-leaf parsley sprigs

2 T chopped dill, optional

½ cup chopped spring onion

1/4 cup extra-virgin olive oil

2 tablespoons water

1/4 teaspoon salt

Make soup:

Melt butter in a 6- to 8-quart wide heavy pot over moderate heat, then cook onion with salt, stirring, until softened, about 8 minutes. Add squash, carrots, potato, and stock and bring to a boil. Reduce heat, then simmer, partially covered, until vegetables are very tender, about 20 minutes. Remove from heat and cool soup, uncovered, 10 minutes.  Working in batches, puree soup in a blender until smooth (use caution when blending hot liquids) and transfer to a bowl. Return pureé to pot and thin with water if desired; simmer 3 minutes. Season with salt.

To make pistou, pulse parsley, optional dill and spring onion in a food processor until finely chopped. With motor running, add oil in a stream, then add water and salt, blending until incorporated.

Swirl 1 tablespoon pistou into each bowl of soup.

For squash soup:

3/4 stick (6 tablespoons) unsalted butter, cut into pieces

1 medium onion, halved lengthwise and thinly sliced crosswise

1/2 teaspoon salt

3-4 yellow summer squash, halved and thinly sliced

2 carrots, thinly sliced

2-3 new potatoes  halved

4 cups chicken or vegetable stock

for pistou:

1 bunch fresh flat-leaf parsley sprigs

2 T chopped dill, optional

½ cup chopped spring onion

1/4 cup extra-virgin olive oil

2 tablespoons water

1/4 teaspoon salt

Make soup:

Melt butter in a 6- to 8-quart wide heavy pot over moderate heat, then cook onion with salt, stirring, until softened, about 8 minutes. Add squash, carrots, potato, and stock and bring to a boil. Reduce heat, then simmer, partially covered, until vegetables are very tender, about 20 minutes. Remove from heat and cool soup, uncovered, 10 minutes.  Working in batches, puree soup in a blender until smooth (use caution when blending hot liquids) and transfer to a bowl. Return pureé to pot and thin with water if desired; simmer 3 minutes. Season with salt.

To make pistou, pulse parsley, optional dill and spring onion in a food processor until finely chopped. With motor running, add oil in a stream, then add water and salt, blending until incorporated.

Swirl 1 tablespoon pistou into each bowl of soup.