Butter a 6-8 cup shallow baking dish. In a small saucepan, heat the butter, garlic, rosemary and pepper, to taste, until the butter is melted. Allow it to sit over very low heat to infuse flavors and keep hot while cooking the potatoes. In a large pot, add the potatoes, cover with water and add a heavy pinch of salt. Bring to a boil over medium heat, then reduce the heat and simmer the potatoes until fork tender, about 15 minutes. Immediately drain the potatoes and then put them back into the hot pot over very low heat, to dry thoroughly. Next, mash potatoes using a food mill and hand masher.
Preheat the oven broiler. Remove the smashed garlic from the warm butter and whisk in about 1 cup of blue cheese. By hand, mix the cheese sauce into the potatoes and season with salt and pepper, to taste. Spread into the prepared baking dish and top with remaining cheese. Broil until the cheese is golden and melting, about 3 minutes. Serve warm!!!