Sweet potato and ginger soup recipe

As requested.
 Skye Gyngell,  A year in my kitchen,
2 large sweet potatoes
2 tbsp unsalted butter
2 red onions
11/2  tbsp grated fresh root ginger
1.5 litres chicken stock (not rabbit!) or water
150ml double cream
1tbsp tamari or soy sauce
1 tbsp maple syrup.
juice of 1/2 lime.
Simplified method; melt butter, fry off onion with salt then add pots and ginger.  Add stock, boil until pots tender, whizz, then reheat and add other ingredients.