Peel the potatoes. Peel the celeriac, using a knife to cut the thick skin away until you are left with only creamy white flesh. Cut the potatoes and celeriac into similarly sized pieces for even cooking.
In a large soup pot, boil the potatoes, garlic and celeriac for 20 minutes until tender in salted water. Add the greens halfway through the cooking time, or at the beginning if you like the greens softer.
Drain, shake and dry with kitchen towels before mashing. Add the butter, milk, cream cheese, and mustard. Season to taste with seasoning and salt and pepper. Sir through before serving.