CELERIAC, SWEETCORN AND LEEK SOUP

1 leek, trimmed • 2 tbsp olive oil • 1 tbsp unsalted butter • 1 onion, sliced • 2 cloves garlic, sliced • 500g Waitrose Celeriac, peeled and thinly sliced • 150ml whole milk • 150g frozen sweetcorn (or drained canned) • 500ml fresh Cooks’ Ingredients Chicken or Vegetable Stock • ¼ lemon, juice • Crusty bread, to serve (optional)