Prepare Beef Cheeks: Trim silver skin and excess fat from beef cheeks using a sharp boning knife. Dice the beef cheeks into approximately 2 cm cubes. Season lightly with cooking salt and leave out for about 20 minutes.
Prepare Vegetables and Herbs: Peel and dice onions (not too small), carrots (1 cm cubes), and celery (smaller dice). Peel, smash, and dice the whole head of garlic. Pick rosemary and thyme leaves and set aside with a bay leaf.
Sear Beef Cheeks: Heat beef tallow in a cast iron pan over medium heat until smoking. In batches, sear beef cheeks on all sides until well browned. Transfer browned beef to a cooking vessel (Dutch oven or gastro tray).
Sauté Vegetables: Add more beef tallow to the pan. Sauté carrots and onions with a pinch of salt for 3-4 minutes until colored. Add celery, garlic, bay leaf, rosemary, and thyme and stir through.
Add Tomato Puree and Wine: Stir in tomato puree, then add red wine to deglaze the pan and help distribute the puree. Pour this vegetable mixture over the seared beef cheeks.
Add Beef Stock and Braise: Return pan to heat, add beef stock and bring to a simmer. Mix well and cover the surface with parchment paper, then two layers of foil. Braise in a preheated oven at 160°C (320°F) for 3 hours.
Make Pastry Dough: Pulse flour, cold cubed butter, and a pinch of salt in a food processor until mixture resembles coarse crumbs. Add ice cold water one tablespoon at a time until dough just comes together. Form into a disc, wrap tightly, and refrigerate overnight or at least 2-4 hours.
Prepare Mashed Potatoes: Peel and halve Desiree potatoes. Boil in salted water until fork tender. Drain well. Mash over low heat with cold butter until butter is emulsified. Adjust consistency with a little milk if desired.
Finish Braised Beef: Remove foil and parchment carefully from braised beef. Drain beef mixture gently through a colander to separate meat from sauce. Return sauce to pan, bring to a simmer, and thicken with dissolved cornflour slurry. Season to taste. Fold beef back into thickened sauce and cool to room temperature, then refrigerate overnight.
Roll Out Pastry and Assemble Pie: Roll out pastry on floured surface to fit a classic pie tin, making one disc to line the tin and a second disc for the top. Chill pastry discs before assembly. Butter the pie tin lightly. Fit bottom pastry into tin, trim edges leaving about 0.5 cm overhang.
Fill Pie and Add Cheese: Fill pastry-lined tin with chilled beef mixture, pressing down to remove air pockets. Sprinkle grated smoked cheddar evenly over the filling.
Top Pie and Seal Edges: Place top pastry disc over filling, trim edges leaving about 0.5 cm overhang. Brush edges with beaten egg and crimp edges tightly using fingers or fork to seal. Brush entire top with egg wash. Cut a steam vent in the top.
Bake Pie: Chill assembled pie in fridge for 10-20 minutes to firm pastry. Bake in a preheated oven at 190°C (375°F) fan forced (or 200°C conventional) for 25-35 minutes until pastry is golden brown and cooked through.
Rest and Serve: Allow pie to rest for 10 minutes before slicing. Serve with buttered mashed Desiree potatoes.