Preheat the oven to 300°F (150°C). Season the brisket generously with salt, black pepper, and BBQ rub, then let it rest for about 10 minutes.

Heat the oil in a cast-iron skillet or oven-safe pot over medium-high heat. Sear the brisket for 4–5 minutes per side, or until deeply browned. Transfer it to a plate.

Add the sliced onion to the same pan and cook for about 5 minutes, stirring occasionally, until softened. Add the minced garlic and cook for another minute.

Return the brisket to the pan and pour in the beef broth. Bring to a gentle simmer, cover tightly, and transfer to the oven.

Cook for approximately 3 hours, or until the brisket is fork-tender. Remove from the oven and let it rest briefly, then chop enough brisket to measure 1 cup for the pot pie.

Increase the oven temperature to 375°F (190°C).

Melt the butter in a medium saucepan over medium heat. Add the flour and whisk continuously for 1–2 minutes until lightly golden.

Slowly pour in the whole milk while whisking. Cook for 4–5 minutes, stirring frequently, until the sauce thickens enough to coat the back of a spoon. Season with salt and black pepper.

Stir the chopped brisket and diced jalapeños into the sauce and cook for 1–2 minutes, just until warmed through.

Remove the saucepan from the heat and fold in the shredded cheese until it begins to melt and the filling becomes creamy.

Spoon the filling into a 9-inch pie dish. Lay the refrigerated pie crust over the top, trim the excess, and crimp the edges.

Cut several small slits in the center of the crust to allow steam to escape. Brush with beaten egg, if desired.

Bake for 25–30 minutes, or until the crust is deeply golden and the filling is bubbling around the edges.

Remove from the oven and allow the pot pie to rest for about 10 minutes before slicing and serving.