Step 1: Preheat oven to 450°F (230°C). Line a baking sheet with parchment paper.
Step 2: Place potatoes in a large pot, cover with water, add 1 teaspoon salt, and bring to a boil. Cook for 15-20 minutes until fork-tender.
Step 3: Drain potatoes and let cool slightly. Place on prepared baking sheet and gently smash each potato with the bottom of a glass or potato masher to about 1/2-inch thickness.
Step 4: Drizzle olive oil over smashed potatoes and season with salt and pepper. Bake for 20-25 minutes until golden and crispy.
Step 5: While potatoes bake, melt butter in a small saucepan over medium heat. Add minced garlic and cook for 1-2 minutes until fragrant.
Step 6: Remove potatoes from oven. Drizzle garlic butter over potatoes, then sprinkle with Parmesan cheese. Return to oven for 3-5 minutes until cheese melts.
Step 7: Garnish with chopped parsley and red pepper flakes if using. Serve immediately.