Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish with a bit of butter or nonstick spray and set aside.

If your frozen shredded hash browns are not yet thawed, spread them out on a tray or in a large bowl and allow them to thaw. Pat them dry with a clean kitchen towel or paper towels to remove excess moisture.

Place a large skillet over medium heat and add 2 tablespoons of unsalted butter. Once the butter has melted and is sizzling, add the diced yellow onion.

Sauté the onion for 4–5 minutes, stirring occasionally, until it becomes soft and translucent. Add the minced garlic and cook for another 30–60 seconds, just until fragrant. Remove the skillet from the heat.

In a large mixing bowl, combine the sour cream, condensed cream of mushroom soup, and whole milk. Whisk or stir until the mixture is smooth and evenly blended.

Stir in the kosher salt, black pepper, smoked paprika, dried thyme, and Worcestershire sauce until the seasonings are well distributed throughout the creamy mixture.

Add 2 cups of the shredded sharp cheddar cheese to the bowl, reserving the remaining 0.5 cup for the topping. Stir to combine.

Add the thawed and dried shredded hash browns to the bowl with the creamy cheese mixture. Gently fold until the potatoes are fully coated.

Add the cooked shredded brisket and half of the chopped fresh parsley to the bowl. Fold everything together gently so that the brisket and parsley are evenly distributed without overmixing.

Transfer the brisket and hashbrown mixture to the prepared 9x13-inch baking dish. Spread it out into an even layer, smoothing the top with a spatula.

In a small bowl, combine the panko breadcrumbs, olive oil, melted butter for topping, and the remaining 0.5 cup shredded sharp cheddar cheese. Toss with a fork until the breadcrumbs are evenly coated and clump slightly.

Sprinkle the breadcrumb and cheese mixture evenly over the top of the casserole, covering the surface from corner to corner.

Place the baking dish on the center rack of the preheated oven and bake for 35–45 minutes, or until the casserole is hot and bubbly around the edges and the topping is golden brown.

Once baked, remove the casserole from the oven and let it rest for 10–15 minutes to set. Sprinkle the remaining chopped fresh parsley over the top for a fresh finish.

Slice or scoop the brisket hashbrown casserole into portions and serve warm, enjoying the tender brisket, creamy potatoes, and crispy cheesy topping together in every bite.