Preheat the oven to 190°C.
Put the celeriac and olive oil in a bowl to combine.
Spread the celeriac out on a baking tray, add the whole garlic bulb and season with salt and pepper.
Place in the oven and roast for 45 - 55 minutes (turning the celeriac a couple of times) until celeriac is golden.
Remove tray from oven and allow to cool for a few minutes.
Slice the top off the garlic bulb.
Squeeze the roasted garlic into a large pan (discarding the skin) and add the roasted celeriac, stock and lemon juice.
Blend until smooth (I use a stick blender).
Heat through, adjust seasoning if necessary and serve with a garnish of snipped chives and some crusty bread.