Preheat the oven to 325 degrees F.
Season the brisket with 2 teaspoons of the pepper and 1 tablespoon of the salt.
Heat a large Dutch oven on high heat.
Add the brisket, fat-cap-side down, and cook, undisturbed, until it has browned, about 3 minutes.
Flip the brisket and cook, undisturbed, until it is browned on the other side, about 3 minutes.
Transfer the brisket to a plate.
Reduce the heat to medium-high.
Add 1 of the yellow onions to the Dutch oven and cook, stirring occasionally, until it has softened, about 5 minutes.
Add the beer to the onion and scrape up any of the browned bits from bottom of the Dutch oven.
Stir the broth, the Worcestershire sauce, and 1/2 teaspoon of the salt into the onion mixture.
Add the browned brisket to the onion mixture and bring it to a simmer over medium-high heat.
Cover the Dutch over with the lid and transfer it to the preheated oven. Cook until the meat reaches an internal temperature of 190 degrees F, is tender, and easily pierced with a fork, about 3-5 hours.
Transfer the Dutch oven from the oven and let the brisket cool in its liquid, covered, for 1 hour.
Increase the oven temperature to 425 degrees F.
While the brisket is cooling, in a large rimmed baking sheet, add the Brussels sprouts, 2 tablespoons of the melted butter, the brown sugar, 1 tablespoon of the salt, and 1/4 teaspoon of the pepper and toss until they are evenly coated. Spread them out into an even layer.
Roast the Brussels sprouts until they start to brown and caramelize, about 10 minutes.
Stir the Brussels sprouts, then roast them until they are crispy on the outside and tender when pierced with a fork, about 5 minutes. Transfer from oven.
While the Brussels sprouts cook, in a 13-inch oven-safe skillet over medium-high, melt the remaining butter until it is sizzling.
Add the mushrooms to the hot butter and cook, undisturbed, until they are deeply browned on their bottoms, about 4 minutes.
Add the remaining yellow onion, the garlic, and 1 teaspoon of the salt to the mushrooms and cook, stirring often, until the mushrooms are deeply browned and tender, about 6-8 minutes.
Reduce the heat to medium.
Add the flour to the mushroom mixture and cook, stirring constantly, until the flour turns golden-brown and smells nutty, about 1-2 minutes.
Transfer the skillet from the heat and stir the remaining salt and the remaining pepper into the mushroom mixture.
Slowly add the milk to the mushroom mixture, stirring constantly, until a thick, gravy-like consistency forms, about 2 minutes.
Reduce the oven temperature to 375 degrees F.
Transfer the braised brisket to a cutting board and cut it into 1-inch-thick chunks. Discard the braising liquid.
Add the caramelized Brussels sprouts and 5 cups of the chopped brisket to the mushroom béchamel and stir to combine. Reserve the remaining braised brisket for another use.
In a large bowl, add the potato tots, the parmesan, and the truffle oil and toss until the tots are coated.
Arrange the potato tot mixture in a circular pattern over the brisket mixture to completely cover the top.
Bake the brisket bake until the potato tots are golden-brown and crispy, about 30 minutes.
Let the brisket bake cool for 10 minutes.
Serve.