Wash, peel and cut the celeriac into small cubes. Do the same with the potato and onion, put the vegetables in a pot with the water, cover with a lid and bring to a boil on medium heat.
Lower the heat to a minimum and simmer for 40 minutes.
Remove the pot from the stove and season with salt and pepper. Add turmeric powder and blend with an immersion blender to obtain a smooth soup.
Fry gently some sage leaves into the olive oil until they get crispy. Dry the leaves on cooking paper and set aside the oil where you fried them, that now has a sage scent.
Divide the celeriac soup into serving plates, top with crispy sage leaves and a drizzle of sage oil and serve immediately.