Celeriac soup with horseradish, lemon and parsley

Ingredients

• 1 small leek, trimmed, cleaned and roughly sliced
• 1 garlic clove, roughly chopped
• 1 medium potato, peeled and diced
• ½ tbsp olive oil
• 1 small celeriac, around 600g, peeled and chopped roughly into 1cm cubes
• 2 pared strips of lemon zest
• 750ml vegetable stock
• 1 x 28g pack flat-leaf parsley, stalks reserved and leaves roughly chopped
• 1 bay leaf
• 2 tbsp freshly grated horseradish (or 2 tbsp creamed horseradish from a jar)
• 1 tsp Dijon mustard
• 250ml semi-skimmed milk
• juice of 1 lemon