Step 1

Preheat the oven to 325 F.

Step 2

Season the brisket on both sides with ½ teaspoon kosher salt and ¼ teaspoon black pepper.

Step 3

In a large Dutch oven, heat the oil over medium-high heat.

Step 4

Add the brisket to the pot and sear until golden brown; flip and repeat.

Step 5

Remove from heat and add the onions.

Step 6

In a glass measuring cup or bowl, whisk to combine the ketchup, stock, garlic, brown sugar, paprika, white vinegar, and the remaining 1 teaspoon salt and ¼ teaspoon black pepper.

Step 7

Pour the sauce over the brisket to coat.

Step 8

Cover and bake for 2 ½–3 hours, or until the brisket is tender. Set aside.

Step 9

Bring a large pot of salted water to a boil over high heat.

Step 10

Add the pasta and cook until almost al dente.

Step 11

Drain and rinse with cold water to stop the cooking process. Set aside.

Step 12

In a large pot, melt the butter.

Step 13

Transfer 2 tablespoons of the melted butter into a bowl and toss with the panko to coat. Set aside.

Step 14

Add the flour to the remaining butter in the pot. Cook, whisking, until the color deepens, 1–2 minutes.

Step 15

Slowly add the milk, whisking until smooth with each addition until all of the milk has been added.

Step 16

Continue to cook over medium heat, whisking, until the mixture is slightly thickened and coats the back of a spoon.

Step 17

Remove from heat. Add the cheeses, salt, and pepper, and whisk until smooth.

Step 18

Add the drained pasta to the pot and stir to coat with the cheese sauce.

Step 19

Preheat the oven to 425 F.

Step 20

Lightly grease a 5-quart baking dish.

Step 21

Pour the pasta mixture evenly into the dish.

Step 22

Top with the panko mixture.

Step 23

Bake for 20–25 minutes, until golden brown.

Step 24

While the mac and cheese is baking, transfer the brisket to a cutting board, reserving the sauce in a bowl.

Step 25

Thinly slice the brisket, and then chop it into bite-sized pieces.

Step 26

Stir the brisket back into the sauce.

Step 27

Pour the brisket over the baked mac and cheese, then garnish with chopped parsley.

Step 28

Serve immediately.