First, roast your veggies. Heat your oven to 350 degrees. Place the onion, leeks, garlic, celery, and celeriac on a large baking tray, drizzle with olive oil, sprinkle with salt and pepper, and lay your thyme sprigs on top all around. Roast in the oven till your veggies are very tender, browned on the edges, and smell amazing. About 30 mins or so.
While your veggies are roasting, fry up the pancetta in a skillet on medium high heat till they are nice and browned and crispy. Remove from the pan into some paper towel to drain the fat.
Remove the veggies from the oven and place in a blender with a cup or two of the chicken stock. Blend well until everything is smooth and creamy. You might need to add a bit more stock or do this in two batches depending on how big your blender is. You can also do this with an immersion blender in a big pot too.
Add everything to a large pot and turn the heat to medium. Add the rest of the stock and bring it to a simmer to heat it through. Turn the heat down to low and add the cheese and heavy cream. Stir until all the cheese is melted. Season to taste with salt and pepper.
Serve with pancetta, chives, and parmesan sprinkled on top. Drizzle a small amount of truffle oil to finish. About half a tsp per bowl or how every much you like!