Preheat oven to 425°F. Toss shallots, garlic, oil, 1/4 tsp. salt and pepper in a bowl. Wrap in a foil packet; roast for 1 hour. Unwrap carefully and let cool. Peel shallots and garlic; puree in a food processor.
Cover potatoes with cold water in a pot; boil. Add 1 tsp. salt and simmer for 15 minutes; drain. Mash potatoes in a bowl. Stir in puree, butter, 1 tsp. salt and pepper to taste. Serve hot.