Preheat oven to 400 degrees F
Slice the top third off each onion and set aside; save tops. Slice an X in the inner 2/3 of each onion. Using a spoon remove the inner 2/3 portion of the onion making a bowl. Dice up the inner section of each onion, on a cutting board, and set aside.
Mix the shredded cheese with the potato salad in a small bowl. Evenly divide the potato mixture into each onion bowl and cover with the reserved onion top. Place on a baking sheet and bake for 20 minutes or until onions are tender.
Bring a medium pot of salted water to a boil over medium heat. Add potatoes and cook for 8 minutes or until slightly tender. Strain, place in large bowl and let cool until slightly warm.
While potatoes are cooking, set up grill or grill pan for direct cooking over medium heat. Place scallions on hot oiled grill and cook for 4 minutes. Transfer to a cutting board and set aside.
To the bowl with the potatoes add mayonnaise, sour cream, mustard, garlic, salt and pepper and gently mix until well blended and potatoes are coated. Slice grilled scallions and mix into the potatoes. Serve immediately or keep covered in the refrigerator for up to 2 days.