Swede and potato ragout

This recipe is versatile regarding texture, it works well as a ragout with soft pieces in a sauce, but works equally as a smooth puree.

1 onion, finely chopped
500 g swede, peeled and chopped (small cubes if aiming for ragout)
400 g potatoes, peeled and chopped (waxy variety works best, small cubes if aiming for ragout)
generous knob of butter
1 leek, white part, prepared and finely chopped
handful of flat-leaf parsley, chopped
100 ml cream
salt, white pepper
1 tsp prepared mustard
1 tbsp soy sauce

In a saucepan, melt butter. Sweat onion for a few minutes, add swede and potato and sweat for a few more minutes. Add water to nearly cover vegetables, bring to the boil and cook over moderate heat for about 20 minutes while stirring occasionally. Add leek and cook for about another 10 minutes, until vegetables are soft. Add cream and season to taste with salt, pepper, mustard and soy sauce.
If aiming for a ragout, this is ready to serve, with chopped parsley sprinkled over.
If aiming for a smooth puree, add parsley, then blend to a smooth consistency, pass through a fine-meshed sieve. Adjust thickness as preferred.

Replace swede by kohlrabi (a relative of swedes). Make a swede-potato-carrot ragout by replacing a proportion of swede and potato by carrots.