The first official day of spring arrives this weekend. It took me many years of living in New England to realize that this is nature’s favorite joke. Here it’s picnic weather one day and winter again the next, which can feel awfully demoralizing when you’re ready for sundresses and wildflowers.

One thing this past year on Earth has taught me: enjoy a good thing while it lasts, and when it gets bad again without warning, make sure you have some soup already made.

Today, the prescription is for my mom’s Green Soup. My mother teaches the kinds of cooking lessons that you don’t soon forget: Always cook with wine that you’d like to drink, because you probably will be drinking while you’re cooking with it. Always be ready to make a cheese plate.

This kind-of vichyssoise was the result of my mom’s legendary foresight in combining two soup recipes from neighboring pages in one of her most-used cookbooks.

It has greens, it has potatoes, it has the comforts of butter and buttermilk. “Oh, and I think I added a little half-and-half, too,” my mother confessed. “Because, of course I did. What don’t I add half-and-half to?”

My mom lives in Albuquerque, N.M., where I grew up — another place with volatile and unpredictable seasons (sometimes it’s 70 degrees during the day there, and 25 at night). When I first moved East, to New York, this soup was one of the things from home that I missed the most. You can eat it warm out of a mug on the couch with a blanket over you, while you curse a late March snow flurry. And you can eat it cold, barefoot on the back porch when it is suddenly lovely outside the next day. You’ll find exactly the way you like it best, but my favorite way is chilled, with a dollop of sour cream, a few snipped chives, and a cold beer.

A few notes about the recipe, which is adapted from volume one of the Pop + Dutch Snackbook: Yes, you are about to cook a cucumber into soup. It is going to feel like a strange, mildly unnatural thing to do. I promise that it is worth doing. You have that cucumber to thank when you notice this soup is refreshing and comforting at the same time. Also, I like starchy russet potatoes best for this, because they break down easily and make the soup silky.

And I like the way chicken stock mixes with the lemon juice and the leeks — the aroma of this combination as it simmers will make anyone in your house hungry and curious, but vegetable stock works well if you want to keep it vegetarian.

In my experience, there is only one way to ruin this soup, and that is to not clean your leeks well enough. Leeks grow up through the dirt. But grit has no place in this soup, so you need to be diligent about this.

When I work with leeks, I rinse them once whole, pulling the leaves back a bit to get the big dirt clumps out. Then, I cut them in half lengthwise (usually cutting off the really tough green parts to save for a stock bag), then slice them crosswise into half-moons. They all go back into the colander like this for a serious rinse, working them through with my fingers to make sure the layers separate and get water between them. Also, rinse your cutting board, please.

Sue’s Green Soup

Makes about 2 quarts

2 Tbsp. butter
4 cups peeled, diced cucumber
2 potatoes, preferably russets
1 bunch scallions
2 leeks
1 bunch fresh spinach
3 cups chicken stock
½ lemon
1 cup half-and-half
2 cups buttermilk
Salt and pepper to taste

Prep the vegetables: peel and dice the cucumber, peel and slice the potatoes, slice the scallions. Rinse and slice the leeks, then thoroughly clean them. Chop the spinach. Juice the lemon half.

Heat butter in a heavy-bottomed stockpot over medium heat. Sauté scallions and leeks in the butter until soft. Add the chicken stock, cucumbers, potatoes, salt, pepper, and lemon juice, and bring to a boil. Reduce the heat and simmer uncovered until potatoes are soft. Add spinach and cook for 2-3 minutes, until wilted but still bright green.

Remove the pot from the heat. If you have an immersion blender, blend the soup up right in the pot until it’s very smooth. If not, transfer the soup, in batches, to a blender and puree.

(Work carefully when blending hot soup: loosen the center cap from the lid and cover the opening with a clean dish towel, which might not be so clean afterwards, but doing this will ensure the soup won’t build up a head of steam and splatter everywhere.)

Return the soup to the pot and add buttermilk and half-and-half. Taste for seasoning. Serve immediately, or, if the sun has come on strong again, chill and serve cold.