Steps

• 1

  Make the mayonnaise: In a small bowl, stir the mayonnaise with the lemon juice, basil, parsley, and cayenne until well mixed. Season to taste with salt and pepper. Cover and refrigerate until ready to use and up to 2 days.

• 2

  Caramelize the onions: Heat the butter in a medium skillet over medium heat. Add the onion, season with a pinch of salt, and cook, stirring occasionally, until wilted and lightly browned, 10 to 15 minutes. Add the garlic and continue cooking and stirring occasionally until the onion is dark brown, another 10 to 20 minutes. (If onion sticks to the pan, splash in a little water.) Set the onion aside at room temperature until ready to serve. (Or cover and refrigerate and let come to room temperature before using.)

• 3

  Make the sauce: In a medium skillet, melt the butter with the pepper over medium-high heat. Add the shallot and garlic and cook, stirring, until softened, about 2 minutes. Add the cognac and cook until almost all the liquid has evaporated. (Alternatively, flambe it.) Add the stock and cream, bring it to a boil, reduce it to a simmer, and cook until somewhat reduced and thickened, another 1 to 2 minutes. Set the sauce aside at room temperature. (You can make the sauce ahead and store it covered and refrigerated for up to 2 days. Reheat it gently before serving.)

• 4

  Sauté the mushrooms: Heat a large skillet over medium-high heat. Add the oil and butter. Add the garlic and cook, stirring, for 30 seconds. Add the mushrooms and thyme. Season with salt and pepper and cook undisturbed until browned on one side, 2 to 3 minutes. Stir and cook until tender and browned all over and tender, 6 to 8 minutes total.

• 5

  Make the sandwiches: When ready to cook, heat the Traeger with the lid closed to 450°F; this will take at least 15 minutes. Holding a leave-in meat thermometer parallel to your work surface, insert it in the middle of one side of the steak. Season the steak generously with salt and pepper.

  450 ˚F / 232 ˚C

  00:04

• 6

  Cook the steak directly on the grill grate with the lid closed until nice grill marks appear on one side, 3 to 4 minutes. Flip and cook until the internal temperature is about 125°F for medium rare. (The internal temperature will increase while the steak rests off the heat.)

  450 ˚F / 232 ˚C

  125 ˚F / 52 ˚C

• 7

  Transfer the steak to a cutting board and let rest for 5 to 10 minutes. Meanwhile, brush the cut sides of the baguette with olive oil and place cut side down on the hot grill grate. Toast until lightly browned.

  450 ˚F / 232 ˚C

  00:05

• 8

  Assemble the sandwich: Spread the herb mayo on the toasted sides of the baguette. Slice the steak against the grain and arrange the slices over the bottom piece of baguette. Top with the mushrooms and the caramelized onions. Drizzle the peppercorn sauce over the onions. Spread the arugula out over everything then top with the other half of the baguette. Cut crosswise into serving portions. Enjoy!