Summer squash are inexpensive and abundant, but little ones can be a little daunted by them. Here, they're whirred into an appealing, pale green soup that strikes the perfect balance between wholesome and yummy. The character of this soup will be determined by what you stir in at the end: buttermilk will make a light and zippy soup, milk a good, mild one, and half and half a rich and creamy one. Served with bread and a salad, it makes a perfect light dinner.
Heat the oil in a soup pot over medium-low heat, then sauté the onion with 1/2 teaspoon salt until it's translucent but not browning, around 5 minutes.
Add the zucchini, garlic, and potatoes, and sauté for another couple of minutes, until the garlic is fragrant and the vegetables are looking a bit less raw.
Stir in the broth and raise the heat to bring the soup to a simmer, then turn the heat back down and simmer gently, stirring occasionally, for 20-25 minutes, until the potatoes are very tender.
Turn off the heat and whir the soup with a stick blender to puree it (or else cool it and puree it very carefully in the blender), then add the buttermilk, milk, or cream, the other teaspoon of salt, and a few squeezes of lemon juice, and blend a bit more.
Taste and re-season as needed, then serve, garnished with herb sprigs or croutons if you like.