Remove papery outer skin from garlic (do not peel or separate cloves); cut top off bulb. Melt 1-1/2 teaspoons butter; drizzle over garlic. Wrap in heavy-duty foil. Place in a 9-in. round baking pan; set aside., Place potatoes and onions in a single layer in an ungreased 15x10x1-in. baking pan. Drizzle with oil; sprinkle with salt and pepper. Toss to coat. , Bake garlic and potato mixture at 400° for 35-40 minutes or until tender, stirring vegetables once. Cool slightly., In a blender, combine 2 cups broth and half of the vegetable mixture; cover and process until blended. Repeat with 2 cups broth and remaining vegetable mixture; set aside., In a large saucepan, melt remaining butter. Stir in flour until smooth; gradually add remaining broth. Bring to a boil; cook and stir for 2 minutes or until thickened. Squeeze softened garlic into pan. Stir in milk and pureed vegetables; heat through.