Peel potatoes and celery root, cut them into large cubes, and set to boil in lightly salted water.
They should cook until very tender, approximately 20 minutes.
Meanwhile, form 4 patties from the meat and season with salt and pepper.
Fry them for 2-3 minutes on each side over high heat, so they get a nice brown crust.
Remove the patties from the heat.
Slice your mushrooms, chop your onions, and cut the leek into thin rings.
First, fry the mushrooms and onions in a bit of extra butter in the pan; fry until they just begin to brown.
Add the leeks and fry for a couple more minutes.
Remove the rosemary leaves from the stem and chop them finely.
Add to the pan along with water, cream, and bouillon cube.
Bring the sauce to a boil and let it simmer for 5 minutes.
Now return the patties to the pan and let simmer for an additional 5 minutes until the sauce thickens slightly and the patties are cooked through.
Season to taste with salt and pepper.
Drain the water from your potatoes and celery root (but remember to save a bit of the water) and place them in a bowl along with the reserved potato water, cream, butter, and dill.
Whisk everything until soft and creamy with a hand mixer.
Season to taste with salt and pepper.
Serve with patties, sauce, and cucumber salad.