In a large heavy bottomed pot, heat up 2 tablespoons EVOO over medium heat and gently sweat onions and garlic until translucent, about 2 minutes. Add the celery root and peeled apple and sauté for another two minutes. Add the milk and water and bring to a boil, then reduce to a simmer and gently cook until celery root is tender and cooked through, about 30 minutes.
Strain and reserve the cooking liquid. Transfer the cooked vegetables to a powerful blender or food processor and add a little of the cooking liquid and process on high, adding more liquid as you go until you have a nice, creamy soup. With the motor running drizzle in the olive oil until fully incorporated. Return the puree to the large pot over low heat and season with salt, pepper and lemon zest. Serve with a sprinkling of toasted hazelnuts, sliced raw apples and celery and a drizzle of EVOO.