Easy seafood chowder

Name: Easy seafood chowder
Ingredients: 1 small potato (about 150g), cut into 2cm cubes; ½ tbsp vegetable oil; 1 celery stick, finely chopped; 1 shallot, peeled and finely chopped; 1 garlic clove, grated; 1 tbsp plain flour; 200ml semi-skimmed or whole milk; 100ml vegetable or fish stock; 80g frozen prawns or smoked fish; 75g fresh or frozen sweetcorn or peas (or use a mixture); 1 small lemon, zested and juiced; ½ tbsp roughly chopped flat-leaf parsley, plus extra to serve (optional); crusty bread, to serve (optional)