Preheat oven to 160°C fan
In a heavy pan, brown the mutton in batches until deeply coloured and set aside
In the same pan, cook the onion, carrots, celery, and garlic until softened and lightly golden
Add the pearl onions to the pan
Stir in the tomato purée and cook for a minute
Add the flour and cook out for another minute
Pour in the red wine and let it bubble and reduce slightly
Add the ale and scrape up all the goodness from the pan
Add the lamb stock, Worcestershire sauce, dried mint, thyme, bay leaf, and mustard
Return the mutton to the pan and season well
Cover and cook in the oven for 3-4 hours until the mutton is tender and the sauce is rich and thick
Remove the bay leaf and transfer to a pie dish, allowing to cool slightly
Top with puff pastry and crimp the edges
Brush with egg wash and cut a small steam hole
Bake at 190°C fan for 30-35 minutes until golden and bubbling
Serve with buttery mash and greens
