Low FODMAP Shepherd's Pie

Name: Low FODMAP Shepherd's Pie
Prep time: 15 min
Cook time: 50 min
Total time: 1 h 5 min
Cuisine: European, British
Ingredients: 1 tablespoon vegetable oil; 500g minced lamb; 1 small stick of celery finely sliced; 1 carrot, diced; Green tops from a bunch of spring onions, finely sliced; A heaped teaspoon plain flour (note: a small quantity of wheat flour such as this is fine); 1 tablespoon tomato purée; 100ml red wine; 200ml water; The leaves from 4 sprigs of fresh thyme; 1 heaped teaspoon English mustard (check the ingredients to ensure there are no high FODMAP additions, I used co-op own brand); Salt and pepper to season; 1kg potatoes (suitable for mashing), peeled and quartered; 50g butter; 50-100ml milk (normal milk should be fine as the dish should feed 4 people but use lactose free milk if you think you'll take a larger portion); Pinch of grated nutmeg (optional)