Cook potatoes in pot of boiling salted water until tender, about 15 minutes. Meanwhile, melt 2 tablespoons butter in heavy large skillet over high heat. Add onions; saute until golden, about 8 minutes. Add garlic; saute 30 seconds. Drain potatoes; return to same pot. Stir over low heat until potatoes appear dry. Transfer to bowl; add 6 tablespoons milk and 6 tablespoons butter. Using electric mixer, beat potatoes until smooth, adding more milk, if desired. Add onion mixture; beat just until blended. Season to taste with salt and pepper.