Preheat grill for low heat and lightly oil the grate.
Mix olive oil and ranch dressing mix together in a small bowl.
Put a potato on a cutting board with a wooden spoon on either side. Make cuts across the potato about 1/4-inch apart, down to the spoons. Repeat with remaining potatoes; place potatoes in a heat-proof dish and add onions.
Spoon oil-ranch mixture evenly over the onions and potatoes. Cover tightly with aluminum foil and place dish on grill. Cook flipping every 10 minutes, until golden brown, about 30 minutes.