Place the sweet potatoes, butter, milk, and salt in a Dutch oven. Cover and cook over low heat for 35–45 minutes, stirring occasionally, until the potatoes are fork tender.
Remove the Dutch oven from the heat. Mash the potatoes with a potato masher until smooth. Season with additional salt and black pepper to taste.
Stir in the crumbled bacon, sliced green onions, and Parmesan cheese until well combined.
Transfer the mashed sweet potatoes to a serving dish. Garnish with the reserved bacon, green onions, and Parmesan cheese.
Serve warm.