Beef:

1. Rub the Dijon mustard all over the rib roast, then season generously on all sides with kosher salt and freshly cracked black pepper.
2. Place on a wire rack set over a baking sheet and roast in a 250°F (121°C) oven until a probe thermometer reads about 115°F (46°C), allowing carryover cooking to bring the internal temperature to 120°F (49°C), about 2-2½ hours.
3. Let rest for at least 30 minutes while the oven preheats to its highest temperature (usually about 500°F/260°C on convection bake, if available). Pat the roast dry with paper towels, then return it to the oven for 3-5 minutes, or until deeply browned.
4. Remove from the oven and let cool to room temperature. Wrap tightly in plastic wrap and refrigerate overnight.
5. The next day, thinly slice the chilled roast as thinly as possible.

Onions:

1. In a large pot (ideally around 10 quarts) set over medium heat, add the butter and olive oil. Once melted and bubbling, add the onions and immediately season with salt. Stir to evenly coat.
2. Cover with a lid and cook, stirring occasionally, until the onions have released most of their liquid, about 15-20 minutes.
3. Remove the lid, reduce the heat to medium-low, and continue cooking, stirring occasionally, until deeply caramelized, about 1-1½ hours.

Jus:

1. In a large saucepot set over medium heat, melt the butter. Add the celery, carrots, onion, and garlic. Season to taste with salt and sauté, stirring occasionally, until softened and fragrant, about 2-3 minutes. Add the black peppercorns and toast until fragrant, about 30 seconds. Stir in the tomato paste until it evenly coats the vegetables. Cook, stirring occasionally, until the tomato paste darkens and caramelizes, about 1-2 minutes.
2. Deglaze with the red wine, scraping up any browned bits from the bottom of the pot. Increase the heat to medium-high, bring to a boil, and reduce by half, about 3-5 minutes.
3. Add the beef stock, thyme, and bay leaves. Bring back to a boil, reduce to a simmer, and cook for 15 minutes. Strain through a fine-mesh sieve, pressing lightly on the solids. Season to taste with salt, Worcestershire sauce, shirodashi, and MSG.

Mayo:

1. In a tall cylindrical container, combine the egg yolks, Dijon mustard, kosher salt, lemon zest, lemon juice, water, oil, and garlic.
2. Using an immersion blender, place the blender head all the way at the bottom of the container and blend until the mixture begins to emulsify. Slowly pull the blender upward until a thick mayonnaise forms.
3. Fold in the horseradish, black pepper, chives, and parsley. Refrigerate until ready to use.

Assembly:

1. Cut the baguettes into 6-8 inch segments and split them open. Toast in a large pan over medium heat with unsalted butter until golden brown. Slather both sides of the bread with the horseradish mayo.
2. Bring the jus to a gentle simmer. Dunk a generous portion of the sliced beef into the warm jus just until heated through, then pile onto the bottom half of the baguette.
3. Top with an even layer of caramelized onions followed by two slices of provolone. Place under a broiler or melt with a kitchen torch until the cheese is melted and lightly browned.
4. Crown your king, serve alongside a ramekin of the warm jus for dipping, and enjoy.