Cook Macaroni according to directions, you want it soft but not too soft. Drain and rinse with cold water thoroughly. Drain well.
Add pasta to large bowl. Add in celery, bell pepper and onion and refrigerate until chilled. 30-60 minutes
Make the dressing. In a bowl, add mayo, mustard, sweet pickle relish, sugar, vinegar, salt, and pepper. Stir until smooth. Refrigerate until ready to use.
Cut up apple into small cubes and add to pasta. Add in frozen peas. Pour the dressing over the macaroni and combine
Cover and refrigerate until ready to serve.
Notes
Great Add-In options
Make it a meal by adding chicken. To do this, use only 8 oz of macaroni and add 8 oz. chicken