Place sage, leek and butter into Thermomix bowl and cook for 7 min/Varoma/speed 1.
Add remaining ingredients and cook for 15 min/100ºC/speed 1.
Check the potatoes are cooked before blending for 1 min/speed 9.
If potatoes are not quite crumbling with the pressure from the back of a fork, cook a further 3 min/100º/speed 1, then proceed with blending.
Adjust seasoning to taste.
Serve with fresh sage sprigs and of course...baguettes!