Lightly coat one side of each of the 16 slices of bread with Rosemary Butter. Set aside. Slice chilled Pot Roast into finger-width slices using a sharp non-serrated knife. Set to the side.
Divide Pot Roast into 8 equal portions on a large cookie sheet. Top each portion of Pot Roast with 1/4 cup Caramelized Onions. Remove reserved Pot Roast liquid from fridge, then skim the fat from the top into a shallow dish. (Return remaining liquid to fridge and save for a soup or gravy base.) Heat a large nonstick skillet over medium-high heat and add 1 to 2 tablespoons Pot Roast fat to the pan. When melted, add the pot roast/onion portions, two at a time, to the skillet. Cook, undisturbed, pot roast-side down, until lightly crispy and browned, about 3 minutes. Flip over and cook until onions are heated through lightly browned, an additional 3 to 5 minutes. Remove to a second cookie sheet and keep warm in oven. Repeat with remaining portions.
Wipe out the nonstick skillet and return to medium heat. Add bread, buttered-side down, 2 slices at a time. Spread 2 tablespoons Horseradish Sauce on the exposed sides of the bread. Divide 2 ounces cheese between the two slices as you would for a grilled cheese sandwich. Cook until bread is browned and lightly toasted and cheese has begun to melt, 3 to 5 minutes. Remove from heat and place on a cutting board. Place 1/4 cup arugula on one side of the bread. Remove one portion of the pot roast/onion from the oven and place on top. Close sandwich. Cut in half and enjoy! Repeat process with remaining ingredients to make 7 more sandwiches.
Place butter, rosemary, salt and white pepper in a small bowl and whisk together with a fork until fully combined. Leave out so that it will remain soft for spreading on bread for sandwiches.
For the pot roast rub: Put granulated garlic, smoked paprika, salt, pepper, crushed red pepper and tarragon in a mason jar and shake until blended. (Unused rub may be stored in an airtight container for up to 1 month.)
For the pot roast: Preheat the oven to 400 degrees F.
Add carrots, celery, onion, garlic and butter to a large roasting pan. Roast until butter is fully melted, about 5 minutes, then stir veg to coat with butter and return to oven. Roast until browned, 10 to 15 minutes. Remove from oven and reduce oven temperature to 275 degrees F.
Coat all sides of pot roast with rub, spanking it into the meat to make sure it properly adheres and helps form a crust. Set to the side. Heat the olive oil in a large nonstick or cast-iron skillet on medium-high heat until beginning to sizzle but not smoking. Sear pot roast on all sides until lightly browned. Take your time with this step, as it is important to build flavor. Transfer roast to pan with prepared vegetables. Arrange vegetables so half are below roast and half are on top. Lay sprigs of rosemary on top of roast. Slowly pour wine over top of roast so as not to displace vegetables and herbs. Scrape remnants of skillet on top of roast. Add just enough beef stock to submerge roast halfway in liquid. Cover pan first with plastic wrap, then top with heavy-duty foil and seal all edges to keep moisture in. (At this low a temp, plastic wrap will not melt and is perfectly safe.)
Place back in oven and cook, undisturbed, for 6 hours. Remove from oven and carefully pull back corner of plastic wrap and foil to let steam release. Remove pot roast from pan and place on a board to rest. Roast should be fork-tender at this point. Using a fine sieve, strain solids from remaining liquid in roasting pan. Discard solids and reserve liquid. Chill roast for at least 1 hour or up to overnight. Pot roast may be made one day ahead of time so that it may be easily sliced for sandwiches.
Add the olive oil or butter to a large nonstick skillet and let melt over medium-high heat until shimmering. Add the onions and use a wooden spoon to lightly coat with melted fat. Spread the onions out in a fairly uniform layer and reduce the heat to medium to medium-low, depending on the strength of your stovetop. The idea is to cook the onions slow and fairly low so that the natural sugar in the onions will release and create a caramelization instead of a standard saute. This will develop a sweet flavor which will contrast beautifully with the other ingredients in the sandwich. This portion takes patience but not a ton of active work. After about 10 minutes, sprinkle salt over onions and stir. Continue cooking, stirring regularly, until the onions are soft and browned completely, about an hour more. (You are looking for the color of a caramel apple, hence the name of the onions.) If you find the onions are sticking, it is perfectly fine to add a bit of water or a little bit more oil or butter so as to prevent burning. Remove from stove and transfer to a small bowl.
Whisk together mayonnaise, horseradish, lemon juice, white pepper, salt and black pepper in a small bowl.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.