Cook the macaroni: Heat a large pot of salted water until it reaches a rapid boil. Boil the elbow macaroni following the directions on the package until it becomes tender. Place in a colander to drain, then rinse thoroughly under cold water to cool. Let it sit until fully cooled—room temperature is fine, or chill it in the fridge if you're short on time.
Prep the mix-ins: In a large mixing bowl, combine the chopped celery, diced onion, sweet relish, chopped hard-boiled eggs, and thawed peas. Gently stir to blend everything together.
Add the cooled pasta: Once the macaroni is completely cool, stir it into the bowl with the veggies and eggs. Be gentle so the pasta holds its shape.
Dress the salad: Mix in some mayonnaise and a small amount of yellow mustard. Mix all the ingredients completely until the salad is evenly coated and has a smooth, creamy consistency. Taste it and add salt as needed.
Chill and store: Transfer the finished salad to a sealed container and refrigerate. It’s best when it’s had time to chill for an hour or more. You can refrigerate this salad for as long as five days, and it will stay fresh.