Scrub potatoes to clean.
Preheat oven to 450 degrees. Line a baking dish (with sides at least 1 inch high) with aluminum foil and spray the foil with nonstick spray.
Place a wooden spoon on either side of the potato. Slice ⅛-1/4-inch slices vertically into potatoes, making the slices down to about ½-inch from the base of the potato.Repeat with remaining potatoes.
Combine melted butter, and crushed garlic in a large bowl and brush over the potatoes, doing your best to get some in between the slits. Don't worry if a lot of the garlic stays behind in the butter. You will get the rest in your final round of brushing the potatoes.
Once coated, place the potatoes on the baking dish and cook in the oven for 30 minutes.
Brush more garlic butter onto the potatoes, making sure to get in between the slices. Cook an additional 25 minutes, or until the potatoes are tender when pierced. Potato cook time will be less with potatoes sliced ⅛-inch and longer for potatoes sliced ¼-inch.
Sprinkle salt, pepper, and Parmesan cheese on top of each potato and bake an additional 5 minutes.
Remove from the oven and top with green onions. Serve.