Poached chicken & pancetta pie with spring onion colcannon

Name: Poached chicken & pancetta pie with spring onion colcannon
Prep time: 30 min
Cook time: 2 h 15 min
Total time: 2 h 45 min
Cuisine: European, British
Ingredients: 1.3-1.4kg whole chicken; 1 litre whole milk; 1 litre chicken stock; 1 onion, halved; 2 bay leaves; 80g prosciutto or pancetta, cut into 1cm pieces; 40g unsalted butter; 40g plain flour; 200ml dry white wine; 1½ tbsp wholegrain mustard; 100ml double cream; 1.5kg Maris Piper potatoes, cut into 2cm cubes; 30g butter; 2 bunches of spring onions, sliced; 100ml whole milk; 30g gruyère or comte, finely grated