Place flour in a medium sized bowl and season well with sea salt and freshly ground black pepper.
Dredge your stewing steak with the seasoned flour and set aside.
Melt half the butter in a saute pan over a medium high heat and sear the meat on all sides until browned but not cooked through. Place the meat in the bottom of the slow cooker.
Add the remaining butter and turn the heat down to medium. Gently saute the onions until they begin to soften. Stir in the mustard seeds, tomato puree, Worcestershire sauce, brown sugar and ale, bringing to the boil and scraping up any bits stuck to the bottom of the pan.
Add the onion mixture to the slow cooker along with the carrots, garlic, bay leaves and thyme.
Set the slow cooker to low and leave for seven hours.
Preheat oven to 230 C.
Spoon the filling into four individual casserole dishes (or one pie dish) and top with the puff pastry. Use a finger dipped in cold water rubbed along the edge of the pie dish to get the pastry to stick to the dish.
Brush with the beaten egg/water glaze and bake for 15 minutes, or until the puff pastry has risen and is lovely and golden.
I realise not all my readers live here in Shetland have access to our fine quality produce, so I urge you to seek out local producers where you live to obtain the ingredients to make this recipe.