Spray a heavy, 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.
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, carrots and celery and sauté for 5 minutes, until beginning to soften.
Add the steak back to the pan along with the garlic and thyme, pour in the remaining stock, the Henderson's relish and red wine stock pot, stir and bring to the boil.
Reduce the heat to low, cover and allow to simmer for 2 hours.
After 2 hours, remove the lid and stir in the mushrooms. Carry on cooking, uncovered, for another 30 minutes or until the steak is soft and falls apart.
Add the crumbled Stilton and stir until melted. Season with some black pepper.
Pour into the 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!