Bring 6 cups water and 1 tsp kosher salt to a rolling boil in a medium pot. Add 2 cups elbow macaroni and cook until just tender (al dente), about 7–8 minutes, stirring occasionally to prevent sticking.

Reserve 1/4 cup of pasta cooking water, then drain the macaroni in a colander and rinse under cold running water until completely cool to stop cooking and chill the pasta. Shake off excess water and transfer to a large mixing bowl.

Dressing

Combine dressing ingredients

In a medium bowl, whisk together 1 cup mayonnaise, 2 tbs apple cider vinegar, 1 tsp sugar, 1 tsp Dijon mustard, 1 tsp celery seed, 1 tsp black pepper, and 1 tsp kosher salt until smooth and emulsified.

Taste the dressing and adjust: add up to 1 tbs lemon juice for brightness or more vinegar/sugar to balance tang and sweetness. If dressing is too thick, whisk in up to 1–2 tbs reserved pasta water to thin slightly.

Mix-ins

Prepare vegetables: finely dice 2 celery stalks and 1 red bell pepper; grate 1 carrot; thinly slice 3 green onions. If using peas, thaw 1 cup frozen peas under cold water and drain.

Add the prepared vegetables and peas to the bowl with the cooled macaroni.

Assembly & Finishing

Combine salad

Pour the dressing over the macaroni and vegetables. Gently fold with a spatula until everything is evenly coated. If the salad seems dry, add up to 2 tbs reserved pasta water or an extra tablespoon of mayonnaise.

Stir in 2 tbs chopped fresh parsley and, if using, 1 tbs fresh lemon juice. Season to taste with additional kosher salt and black pepper.

Cover the bowl and refrigerate at least 1 hour, preferably 3–4 hours, to let flavors meld. Stir again before serving and adjust seasoning if needed.

Serve chilled. Keeps well in an airtight container in the refrigerator for 3–4 days.