Peel, wash and roughly chop the potatoes. Peel celeriac and turnip and chop coarsely. Cook everything in boiling salted water for about 20 minutes
Drain the cooking liquid, add butter to the vegetables and mash coarsely. Season to taste with salt and pepper. Put into a greased casserole dish, smooth it down and braid the bacon slices on top to form a grid
Bake in a preheated oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: see manufacturer) for approx. 20 minutes