Preheat the oven to 450 degrees.
Place the pepper halves on a baking sheet and brush with olive oil and sprinkle with salt. Roast until the skin is blistered, about 15 minutes. When cool enough to handle, remove the peels, chop, and place in a small bowl. Set aside.
Shred the cheese and place in a large bowl. Add the mayonnaise, red onion, garlic, black pepper, and peppers. Fold the mixture until the pimentos are mixed throughout the cheese and the mayonnaise has binded with the cheese.
Stuff each pepper half with about 2 teaspoons of pimento cheese mixture. Sprinkle with black pepper. Refrigerate until ready to serve.