1medium onion, chopped or 1 leek, halved lengthwise and sliced (white and light green part only)
8ounces(225 grams) white or yellow potatoes, peeled and diced into ½-inch cubes(about 2 small potatoes or 1 large)
Kosher salt and freshly ground pepper
½cup(75 grams / 2.6 ounces) elbow macaroni
½cup(75 grams / 2.6 ounces) thawed and squeezed frozen chopped spinach(about 175 grams / 6 ounces before thawing and squeezing or about 225 grams / 8 ounces raw spinach, chopped)
1cup(85 grams / 3 ounces) grated Gruyere cheese
Heat the butter over medium heat in a medium Dutch oven or pot. Add the onion and cook until softened, about 5 minutes. Then add the diced potatoes and stir for 20 to 30 seconds.
Add the vegetable stock, raise the heat to high, and bring to a boil. Season with salt and pepper, reduce the heat to low, partially cover, and simmer for 8 minutes.
Add the macaroni and continue to cook until the macaroni and potatoes are both tender, about 8 minutes more. Stir in the spinach, heavy cream, and grated Gruyere. Make sure to stir well so the cheese melts into the broth evenly. Adjust seasoning as needed and serve immediately.
To thaw the spinach I like to put it in a fine mesh strainer and briefly run warm water over it. Then squeeze it tightly in handfuls over the strainer to press out excess water. There are measurements above for weighing the frozen spinach before and after thawing and squeezing. This doesn't need to be super exact, but there is a pretty big difference in weight and volume of frozen spinach before and after thawing/squeezing.
If you use fresh spinach instead be sure to use the weights provided above, chop the spinach and then stir it in at the same time as directed in the recipe for frozen spinach. It will look like a lot but will wilt down rather quickly in the hot broth.
If desired, add additional diced vegetables like carrots or turnips. Add carrots with the onions and turnips with the potatoes. In this case you may want add a little extra stock or milk to increase the amount of broth to make up for the extra vegetables.
If making this soup in advance, do not cook the macaroni directly in the soup as it may get soft and mushy over time. Instead make the soup without the macaroni and cook the macaroni separately in boiling water. Then stir in the cooked macaroni before serving to heat through, or add it to individual bowls and pour piping hot soup over the top.
If you make the soup as directed and have leftover soup with macaroni already in it, that's fine. The texture of the macaroni in the soup will change by the next day, but it will still taste delicious! You may want to slightly thin out the soup with a bit more broth, water, or milk when you reheat it.