Coat the round roast in the 2 tablespoons of salt and cracked pepper. Let it sit, uncovered, in the fridge overnight.
Heat a covered sous vide water bath to 135F degrees.
Heat a skillet over high heat until smoking. Add the round roast, fat cap side down, and sear for 2-4 minutes, until golden brown.
Flip over and sear the other side for 2-4 minutes, until a crust has formed. Continue for all sides until a brown crust is formed all around.
Remove and set aside to cool.
Turn the heat in the skillet down to medium and add the red wine. Cook down for 2-3 minutes, using a wooden spoon to scrape any bits off the bottom of the pan.
Add the peppercorns, garlic and bay leaves and continue to simmer for about 2-3 minutes.
Add the beef stock and simmer for 2-3 minutes.
Add the additional ½ teaspoon of salt to taste if required (taste it first - if you used homemade unsalted stock, you will need to add salt here).
Turn the heat off and let the mixture cool so it won't melt the bag.
Combine the beef stock mixture and the roast in a vacuum seal or ziplock bag (if using the water displacement method discussed above). **It may be easier to use the water displacement method here as we are adding liquid to the bag.
Vacuum seal the bag using the moist setting on the vacuum sealer, OR seal the top of the ziplock bag, leaving the corner open while submerging the bag so the air is pushed out. Seal once the bag is submerged.
Cook the roast for 24 hours, checking periodically to ensure too much water has not evaporated, and that your beef is remaining completely submerged.
During the last 10-15 minutes of the beef cooking, heat a skillet over medium heat and add the butter and onions.
Sauté for 5-7 minutes, until the onions have browned and softened. Remove and set aside.
Remove the bag from the water bath and take the meat out of the bag, reserving the jus.
Line a strainer with cheesecloth and strain the jus. If you don't have cheesecloth, you can just use a strainer, but the jus won't be as clear. **You can strain it multiple times for extra clear jus.
Use a sharp knife to thinly slice the roast.
Place the beef on a French roll, add the cooked onions, top with cheese and place under the broiler for a minute or two to melt the cheese.
Slather it in horseradish sauce (see below) and serve with the jus on the side.