Cut the brisket into 1/2-inch cubes and sear them in a hot skillet over medium-high heat until browned on all sides, about 8-10 minutes total. This develops deep flavor through the Maillard reaction—a crucial step that makes the casserole taste restaurant-quality rather than bland. While the brisket cooks, finely dice the onion and mince the garlic, then set aside. Peel the potatoes and immediately shred them into a clean kitchen towel, then squeeze firmly to remove as much moisture as possible—this is the secret to getting crispy hashbrowns instead of mushy ones. I always squeeze harder than I think necessary because the drier the potatoes, the better the texture.

In a large mixing bowl, combine the sour cream, cream of mushroom soup, melted butter, minced garlic, salt, black pepper, smoked paprika, and Worcestershire sauce. Stir until completely smooth and uniform. The butter should be cooled slightly so it blends evenly without clumping. This sauce is the binding agent that keeps everything moist while baking, so make sure there are no lumps of soup remaining.

Preheat your oven to 350°F. Spread the shredded potatoes evenly in a 9x13 inch baking dish, breaking up any clumps with your fingers. In a large bowl, combine the cooled seared brisket from Step 1, diced onion, 2.5 cups of the sharp cheddar cheese, and pour the creamy sauce mixture from Step 2 over top. Fold everything together gently but thoroughly until the potatoes are coated and evenly distributed. The sauce will seem thin at first, but it will thicken as the potatoes release their starches during baking.

Transfer the potato mixture to your prepared baking dish if not already done, spreading it into an even layer. Sprinkle the remaining 1 cup of sharp cheddar cheese over the top, making sure to cover most of the surface. Bake uncovered at 350°F for 45 minutes to 1 hour, until the top is golden brown and the edges are starting to crisp. I like to check it at the 45-minute mark—if the potatoes feel tender when pierced with a fork, it's done; if they still feel firm, give it another 10-15 minutes. The timing depends on your oven and the exact thickness of your layer.

Remove the casserole from the oven and let it rest for 5 minutes—this allows the sauce to set slightly so it won't be too loose when served. Scatter the finely chopped cilantro (including stems for extra flavor) over the top. Cut into squares and serve warm. The cilantro adds a fresh, aromatic brightness that cuts through the richness of the cheese and sour cream.