In a large pot, cover potatoes with cold water.
Bring the potatoes to a boil, reduce heat and simmer until the potatoes are tender when pierced with a fork, about 15-20 minutes.
Drain potatoes and rinse with cold water. Once the potatoes are cool, cut them into small chunks, about 1/4-inch pieces. Remember you're stuffing this salad into a mini bell pepper, so the pieces need to be someone smaller than usual.
Place potatoes in a large bowl and set aside.
As the potatoes are cooling, you can prepare the bell peppers.
Lay the peppers on a cutting board and using a sharp knife, slice horizontally through the upper third of each pepper.
Reserve the pepper pieces to include in the salad.
Rinse and remove the seeds and membranes from inside each pepper and dry them thoroughly.
Set the prepared peppers on a baking sheet or serving tray and set aside.
Now, prepare the salad.
In a bowl, whisk together the mayonnaise and mustard.
Pour the mix over the cooled potatoes and gently stir until the potatoes are well coated.
Add the onions, celery, pickle relish, and eggs.
Finely dice the extra pieces of bell peppers and put those in the bowl as well.
Stir gently to combine.
Season with salt and black pepper, to taste.
Using a spoon, carefully fill each pepper shell with potato salad.
After all the shells are full (you will have extra potato salad - enjoy that as you wish, or fill more peppers!) garnish with extra green onions!
Serve immediately or chill in the refrigerator for up to 2 days.