Place garlic on small piece of aluminum foil and drizzle with 2 tsp oil. Fold up the foil and set aside.
Place pepper on small parchment-lined baking pan and drizzle with remaining oil. Put the garlic next to it on the pan. Roast at 475° for 20-30 minutes until pepper is charred and soft. Remove from oven and let cool.
While those are roasting, add potatoes to a pot filled with cool, salted water. Bring to a boil over high heat then lower heat and simmer until tender, about 25 minutes. Drain and add back to the pot. Set aside.
While the potatoes cook, and while still warm, peel skin from pepper. Squeeze garlic cloves from bulb. Place both in the pot with the potatoes.
Season with salt & pepper, add the butter, and mash to your desired consistency using a masher or immersion blender. Serve immediately.
