Roast the Garlic: Preheat oven to 400°F (200°C). Slice off the top of the garlic bulb to expose the cloves. Drizzle with olive oil, wrap in foil, and roast for 30–35 minutes until soft and golden. Cool slightly, then squeeze roasted garlic out of skins and mash into a paste.
Cook the Potatoes: Place peeled and cubed potatoes in a large pot, cover with cold salted water. Bring to a boil, reduce heat, and simmer for 15–20 minutes, until fork-tender. Drain well.
Mash the Potatoes: Return potatoes to the pot or a mixing bowl. Add roasted garlic paste, butter, heavy cream, and sour cream (if using). Mash until creamy and smooth. Season with salt and pepper to taste.
Make the Brown Gravy: In a medium saucepan, melt butter over medium heat. Stir in flour to form a roux, cooking 1–2 minutes until lightly golden. Gradually whisk in broth, stirring constantly to avoid lumps. Add Worcestershire sauce, onion powder, salt, and pepper. Simmer for 5–7 minutes until thickened and glossy.
Assemble: Serve a generous scoop of garlic mashed potatoes, ladle warm brown gravy on top, and garnish with fresh herbs (optional).