Bring a large pot of water to a boil; add potatoes. Boil 20 minutes or until fork-tender. Drain and set aside to cool slightly.
Preheat oven to 450° F. Line a rimmed baking sheet with parchment paper and brush with olive oil.
Transfer boiled potatoes to the baking sheet, giving some space between each. Oil the base of a sturdy glass with olive oil or cooking spray, then gently press the potatoes until they are approximately ½ inch thick.
Brush the smashed potatoes generously with the melted butter. Sprinkle with garlic, salt, and pepper.
Bake for 20-25 minutes, or until potatoes are crispy and browning at the edges. Remove from oven, and sprinkle generously with chopped dill, green onions, and additional salt and pepper, if desired. Serve while warm.