To make the foil packs, tear two ~12" x 20" sections of foil. Fold each in half. For each piece, roll the sides adjacent to the fold tightly 3–4 times to make a pouch.
Mix the sliced onions and peppers. Divide approximately between the two foil pouches. Add 1/2 tsp of salt, 1/4 tsp of freshly ground black pepper, and 1 tbsp of oil to each pouch. Fold the top of the pouch several times to seal. Refrigerate until use.
Prepare charcoal grill for indirect heat. When the coals are hot and completely ashed over (white), add the sausages directly over the coals. Place the pepper-onion packs on the indirect heat side of the grill.
Grill the sausages for 3–5 minutes each on 2 sides, or until browned (check to be sure they don't burn; see Recipe Note #1). Continue grilling, turning as necessary, until thoroughly cooked, an additional 8–9 minutes (total 15–18 minutes) to cook through. If you'd like to toast your rolls, move the sausages to indirect heat and toast the bread until lightly browned.
To assemble the sandwiches, lay 1–2 slices of provolone cheese (if using) on the bottom of the roll. Add one hot sausage and ~1/2 cup of the pepper-onion mixture (see Recipe Note #2).