Carefully wash and scrub the potatoes to remove any dirt. Pat them dry with a towel.
Place each potato on a cutting board and use a sharp knife to make several evenly spaced slices, about 1/4 inch apart, stopping just before you reach the bottom to keep the potato intact.
In a small bowl, combine melted butter, minced garlic, chopped rosemary, salt, and black pepper.
Using a pastry brush or spoon, generously baste the garlic butter mixture over the potatoes, making sure to get between the cuts.
Place the prepared potatoes on a baking sheet lined with parchment paper and drizzle any remaining garlic butter over them.
Bake in the preheated oven for 45-55 minutes, or until the potatoes are tender and crispy on the edges.
Baste with more garlic butter halfway through the cooking process for extra flavor.
If using, sprinkle grated Parmesan cheese over the potatoes during the last 5-10 minutes of baking to allow it to melt and get golden.
Once done, remove the potatoes from the oven and garnish with fresh parsley before serving.
Notes
For extra flavor, baste with garlic butter halfway through baking.