Cut off pointed end of each garlic bulb; place garlic on a piece of aluminum foil, and drizzle with oil. Fold foil to seal. Bake at 350° for 1 hour or until very soft; cool. Squeeze pulp from garlic bulbs to measure 3/4 cup. Reserve any remaining garlic for another use. Process 3/4 cup garlic, milk, and next 4 ingredients in a blender or food processor until smooth; transfer to a large bowl.
Peel potatoes and slice very thinly. Add potatoes to milk mixture as they are sliced. (This keeps potatoes from changing color.) Layer one-third each of potato mixture, herbs, and cheese in a greased 13" x 9" baking dish. Repeat layers twice, ending with cheese. Bake, covered, at 350° for 45 minutes. Uncover and bake 45 more minutes or until browned and bubbly. Let stand 10 minutes before serving.