Stock really makes the difference in this dish.

3 T extra-virgin olive oil. divided
3 c peeled and cubed celeriac
2 c cubed potatoes
salt and freshly ground black pepper
1 c chicken or vegetable stock
2 t lemon juice, or to taste
1 T butter

1. Warm 2 T of the olive oil in a large skillet over medium heat. Add the celeriac and potatoes, sprinkle with salt and pepper and saute until beginning to brown, 5 to 7 minutes. Add the stock and any brown bits that have stuck to the skillet (there may be a lot).

2. Bring to a simmer and cover. Cook until the vegetables are tender, around 10 to 12 minutes. Uncover and mash the vegetables in the skillet, stirring them to soak up the pan juices. Stir in the lemon juice and the remaining 1T of olive oil. Taste for seasoning, adding more salt, pepper and lemon juice, if desired. Stir in the 1 T of butter. Serve hot.