We eat these quite a lot during the winter, particularly in place of potatoes.
For each person, peel and trim one to two parsnips, depending on their size. Halve and slice about 1/4″ thick (think carrot coin size). Uniform thickness means even cooking, i.e. all the pieces will cook in the same amount of time. Steam until just soft in salted water or over a pot of something else boiling, like soup or rice. Bang them into a pan to glaze with butter to taste, season with salt and pepper, and serve warm.