Trim the fat cap and any visible thick fat off the roast. Reserve for the tallow below.
Salt the roast liberally on all sides and place on a rack in the refrigerator overnight to dry brine.
The next day, place the roast in a vacuum bag with rendered beef tallow and seal.
Cook at 131°F / 55°C for 8 hours, then chill in an ice bath and refrigerate.
Place all the trim in a vacuum bag with herbs, smashed garlic clove and peppercorns.
Cook the fat at 183°F / 83°C for 8 hours
Strain the rendered fat through a fine mesh strainer, then chill completely in the fridge.
In a saucepan over medium heat, combine the tallow and flour to make a roux. Cook 1 - 2 minutes.
Add the red wine, whisking well to incorporate into the roux. Cook for 1 - 2 minutes until the strong alcohol smell dissipates.
Add the beef stock and whisk well to combine until there are no clumps. Bring to a simmer, then lower the heat.
Add the garlic powder, porcini powder and worcestershire sauce and whisk to combine.
Add salt to taste, then remove from the heat. The jus can now be refrigerated until ready to use.
Set your oven to 250°F / 121°C. Place rolls in the oven to heat while you finish the dish.
Remove the meat from the fridge. It's important to slice while cold so you get a more consistent slice.
Using a meat slicer, slice roast to 1.5 - 2mm thick. Alternatively, slice as thin as possible with a knife.
While slicing the meat, heat the jus over medium heat until it reaches 130°F / 54.5°C. The Breville Commercial Control Freak probe mode makes this super easy - but if you don't have one, use a stove and monitor the temperature to make sure it doesn't get too hot. If you'd like, you can skim off the fat while the jus is still cold - this is personal preference.
Once the meat is sliced, place in the jus for 1 - 2 minutes to heat through.
Meanwhile, mix mayo and Creamy Parisian sauce to create a sandwich spread.
Remove the bread from the oven and slice open.
Spread the Creamy Parisian spread on both halves of the bread
Using a slotted spoon, remove the meat from the jus and add 6 - 8 ounces to each sandwich.
Once the sandwich is assembled, slice it in half on the bias.
Serve sandwich with a side of the jus for dipping.