Preheat oven, prep garlic: Preheat oven to 400 degrees. Cut garlic heads about 1/2-inch from the top to expose cloves.
Roast garlic: Transfer garlic heads to a sheet of foil, drizzle 1 tsp oil over each head, wrap foil tightly around garlic so it's fully covered. Roast in preheated oven until lightly golden and cloves are soft, about 40 - 45 minutes.
Mash garlic: Remove from oven and let cool (cook potatoes while they cool) then press and pinch cloves out of peels into a small bowl. Mash cloves with a fork and set aside.
Cook the potatoes: Place potatoes in a large enameled cast iron pot and add enough cold water to cover potatoes, sprinkle in 1 Tbsp salt. Bring water to a boil over medium-high heat, then cover with lid, reduce heat to medium-low and allow to boil until potatoes are very tender when pierced with a fork, about 20 minutes.
Heat milk, cream and butter: Near the end of potatoes cooking, measure out milk in a liquid measuring cup, add cream and butter. Heat in microwave until hot, about 45 - 60 seconds.
Drain and mash potatoes: Drain potatoes then return to pot and mash with a potato masher (they should have some lumps in them).
Stir in milk mixture and garlic: Add hot milk mixture and mashed roasted garlic, and stir with a spoon while seasoning with salt and pepper to taste. Serve immediately with more butter and garnish with parsley if desired.