Bring a pot of salted water to the boil.
Add first measure of celeriac and potato to pot of boiling water. With the lid on cook for 12- 15 minutes or until soft. Drain well, return to pot and add butter and milk, mash with a potato masher or use a hand blender for a smooth puree.
While celeriac and potato are cooking prepare the rest of the meal. Remove stems from silverbeet and thinly slice till you have 3 cups worth, grate courgette and roughly chop garlic, set aside.
Pat lamb dry with paper towels, rub with cumin, salt and rosemary. Heat a drizzle of oil in a fry pan on high heat, cook lamb 2-3 minutes each side, for medium, or until cooked to your liking. Remove from pan and cover in foil to rest for 3 minutes, before slicing thinly against the grain.
Keep pan on heat, reduce heat to medium, add a drizzle of oil if needed. Add all sautéed silverbeet ingredients and cook for 2-3 minutes, scraping any meat spices from the bottom of the pan. Season with salt and pepper.
Whisk mayonnaise, vinegar and mustard together in a large bowl. Grate second measure of celeriac or cut into matchsticks, add to bowl and toss to coat. Season with salt.
To serve, spoon celeriac mash onto plates and top with sautéed silverbeet. Divide lamb loin between plates and finish with some celeriac salad.