Combine first 3 ingredients in a large Dutch oven; bring to a boil. Reduce heat, and simmer 25 minutes or until tender. Remove parsnips from the pan using a slotted spoon, reserving 1 1/2 cups cooking liquid. Place parsnips in a food processor. With processor on, slowly add reserved 1 1/2 cups cooking liquid; process until smooth. Add butter, salt, pepper, and garlic; process until combined. Spoon mixture into a bowl; stir in oil.