Cut the potatoes into 1-inch cubes and place them in a large stock pot. Fill the pot with cold water until it is one inch over the top of the potatoes. Set the pot over high heat and bring to a boil. Once boiling, add 1 tablespoon salt. Then set the timer and cook the potatoes for 5 minutes. After 5 minutes, stir in the macaroni noodles and continue boiling for 6-8 minutes, until the pasta is al dente.
Meanwhile, in a medium bowl mix the mayonnaise, sweet pickle relish including juices, apple cider vinegar, mustard, allspice, 1/2 teaspoon salt, and pepper to taste. Stir until smooth. Then use a grater to shred the onions and carrot.
Drain the potatoes and macaroni in a colander and place in a large salad bowl. Add in the onion, carrots, peas, and scallions. Pour the dressing over the top and mix until well combined.
Cover the potato macaroni salad and refrigerate for at least 4 hours. If you have time to make it ahead, it tastes even better on day two! Keep refrigerated in an airtight container for up to one week.
NOTES: This salad is sometimes made with unpeeled new potatoes for a slightly different texture. Follow the instruction as is, yet leave the potato skins on when you chop them.