1. Place celeriacs in a bowl. Cover with cold water; stir in lemon juice. Let stand while preparing remaining ingredients. Drain when ready to boil.
2. Place potatoes and celeriacs in separate pots; cover with cold water. Add a large pinch of salt to each pot; bring to a boil over high heat. Reduce heat to medium-low; cook until vegetables are very tender, about 30 minutes for potatoes and 15 to 20 minutes for celeriacs.
3. Drain celeriacs. Pulse in a food processor until smooth. Drain potatoes, return to pot and add celeriacs, milk and butter. Mash with a potato masher or put through a ricer. Season with salt and pepper. Serve.