Peel the turnips, cut into slices and cook with 1.5 teaspoons of salt and water - enough to cover them. Also process the potatoes and cook them in an extra saucepan with 1.5 teaspoons of salt and water.
When both are done, put them together in a mixing bowl to puree. Catch the stock juice from the turnips and simply pour the water away from the potatoes.
Melt 2 tablespoons of butter in one of the pots and stew a chopped onion in it. Make a roux with flour and deglaze with the scalding water. Add the stock cube and let everything simmer until the onion is soft, then fold the turnip potato puree into the roux and season with salt, pepper and nutmeg.
A cold meat sausage goes perfectly with it!
I boil the turnips in advance, then use the broth and just pour water on them.