Cut the potatoes into even 1/2-inch cubes, slice the onion, and dice the pepper.
Toss the vegetables with olive oil, garlic, Italian seasoning, salt, and pepper.
Divide among four 18-inch sheets of heavy-duty foil and top with butter.
Fold the long sides together twice and fold in the ends twice, leaving space for steam.
Cook on the grate 25 to 35 minutes, flipping every 10 minutes.
Check that a fork slides into the potatoes easily.
Rest 2 to 3 minutes, open carefully, and top with parsley and Parmesan.