Easy fish chowder

Name: Easy fish chowder
Ingredients: 1 tbsp olive oil; 200 g streaky bacon, rind removed; 2 onions, finely chopped; 2 celery stalks, thinly sliced; 1 garlic clove, crushed; 1 tbsp plain flour; 500 ml milk, hot; 350 g desiree or pontiac potatoes, peeled, cut into 1cm chunks; 375 ml fish stock; 100 g canned corn kernels, drained; 500 g boneless white fish (such as perch or cod), cut into bite-size chunks; 24 mussels, scrubbed, debearded (optional); 150 ml thick cream; 2 tbsp chopped flat-leaf parsley, to garnish