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 hot pot over very low heat, to dry thoroughly. Next, pass them through a food mill over a bowl. If you don't have a food mill, you can hand mash or use a ricer.
Preheat the oven broiler.
Remove the smashed garlic from warm butter and whisk in about 1 cup of the blue cheese. By hand, mix the cheese sauce into the potatoes and season with a little 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. Remove the potatoes from the oven and serve.