In a Dutch oven or similar large pot, melt the butter with the oil over medium heat. Add the onion and cook, stirring occasionally, until softened, about 5 minutes—reduce the heat as needed so the onion doesn’t brown. Stir in the garlic, salt, and pepper and let cook 30 seconds.

Add the zucchini and 3 cups of the broth. Bring to a boil, then reduce the heat to a simmer. Let simmer until the zucchini is tender, about 20 minutes. Stir in the spinach and let wilt.

Add the beans and tender herbs. With an immersion blender, puree the soup until smooth (you can also carefully transfer it to a blender and puree in batches. Be careful not to overfill the blender, leave the pour hole open, and lay a towel over the top to guard from hot splatters).

Stir in the Parmesan and lemon juice. If the soup is thicker than you would like, add the remaining broth until you reach your desired consistency.

Place the Greek yogurt in a bowl and add a few spoonfuls of the soup to it; stir to combine, then return the mixture to the pot and stir until smooth (this keeps the Greek yogurt from curdling). Adjust the seasoning as desired—depending upon your broth, you may want a pinch or two of additional salt. Serve hot, with a dollop of additional yogurt and sprinkle of herbs.