How to Make It
Place sweet potatoes in a large saucepan; cover with water to 2 inches above potatoes. Bring to a boil over high; reduce heat to medium-low, and simmer until tender, about 18 minutes. Remove from heat. Drain and return potatoes to saucepan.
Combine butter and olive oil in a small skillet over medium; cook until browned and fragrant, about 3 minutes. Remove from heat; stir in thyme.
Add milk, salt, and pepper to sweet potatoes; mash with a potato masher to desired consistency. Drizzle with butter mixture; gently stir once or twice. Garnish with thyme leaves.