Celeriac Mash

2 celeriac, peeled and cut
2 potatoes, peeled and cut
1 onion, diced into large pieces and divided in half
2 tablespoons butter

In a small sauce pot place celeriac, add water to cover.
In separate pot place potato with water to cover.
Place half the onion in each pot and simmer until each is fork tender.
Strain liquid and place all ingredients, including butter, in a food processor or blender.
Puree until mash has a smooth consistency.
Season with salt and pepper to taste.