Spray a medium saucepan with low calorie cooking spray and place over a medium-high heat.
Coat the steak in the flour and add to the hot pan. Cook for 3-4 minutes until browned on all sides. Remove from the pan and add to the slow cooker.
Deglaze the pan with a little stock, scraping up and mixing in any bits off the bottom of the pan, and then spray again with low calorie cooking spray. Add the onions, garlic and thyme and saute for 5 minutes.
Add the onions to the slow cooker, along with the carrots, celery, remaining stock, stock pot and Henderson’s relish.
Cook for 4 hours on high or 7 hours on low.
Half an hour before the end of cooking time, stir in the mushrooms and cook for the remaining 30 minutes.
Add the crumbled Stilton and stir until melted. Season with some black pepper.
Pour into an ovenproof dish, and preheat the oven to 180°C.
Cut the filo sheets into strips, scrunch up and place on top of the pie, covering the filling completely. Use a pastry brush to egg wash the top.
Place on a baking tray to catch any spills in case the filling bubbles over and pop in the oven for 15 minutes, or until the pastry is golden and crisp. Serve!