Start by bringing a large pot of salted water to a rolling boil. Add elbow macaroni and cook according to package instructions until al dente, usually around 8-10 minutes. Once finished, drain the pasta and rinse it under cold water for about 1-2 minutes to stop the cooking process and cool it down quickly.

While the pasta is cooking, prepare the broccoli by bringing a smaller pot of salted water to a boil. Carefully add the chopped broccoli florets and blanch them for 1-2 minutes until they are bright green and tender-crisp. Immediately transfer the broccoli to a bowl of ice water to halt the cooking, then drain well after a few minutes.

In a large mixing bowl, combine mayonnaise, sour cream, apple cider vinegar, sugar, salt, and pepper. Use a whisk to blend these ingredients together until the mixture is smooth and well incorporated, creating a creamy dressing.

Once the pasta has cooled completely and the broccoli is drained, add the pasta, blanched broccoli, shredded cheddar cheese, and diced red onion to the bowl with the dressing. Gently toss everything together until all the ingredients are evenly coated.

Cover the bowl with plastic wrap or transfer the salad to an airtight container. Place it in the refrigerator and let it chill for at least 1 hour.

Before serving, give the pasta salad a quick stir to refresh the ingredients. Taste and adjust the seasoning with additional salt or pepper as needed.