Parsnips would make an excellent substitution. The puy lentils from France are best for salads as they tend to hold their shape when cooked; regular green or brown lentils are perfectly fine, they take a bit longer to cook is all. Boiling the whole celeriac first makes it easier to peel.
1 celeriac root (about 1 to 1 1/2 lbs)
1 t salt, optional
1 c French puy lentils; or regular green or brown lentils
6 T extra virgin olive oil
3 T white wine vinegar
1 1/2 t minced garlic
1 T minced fresh herb, or to taste
large pinch of ground nutmeg
salt and pepper, to taste
1/2 c crumbled blue cheese, or feta cheese
1. Rinse and scrub celeriac. Put in a pot and cover with water. Add salt, if you like. Bring to the boil and cook until celeriac is tender, about 45 minutes. Drain, cool, peel. Cut enough of the celeriac into 1/4-inch dice to yield 1 1/2 cups. Alternatively, peel celeriac, cut into the dice and cook in (salted) boiling water until tender.
2. Meanwhile, cook lentils in about 3 cups of boiling water to for about 20 minutes, until tender but still firm to the bite. Regular green or brown lentils will take about 30 minutes. Drain, cool.
3. Whisk oil, vinegar, garlic, herb, and nutmeg together in a small bowl. Season with salt and pepper. Stir in cheese.
4. In a serving bowl, mix diced celeriac, lentils and dressing. Taste for seasonings. If not using immediately, let stand at room temperature, tossing occasionally.