Set your sous vide water bath to 132°F.

Place hanger steak into a large zip-top bag. Add a few pats of butter to the bag. Seal the bag and sous vide the steak for 2 hours.

Remove the steak from the bag and pat dry. Heat oil in a pan or flattop over high heat. Season the steak with salt and pepper. Sear on all sides until a deep brown crust forms, about 2-3 minutes per side.

Remove from heat and let rest for 15-20 minutes before slicing.

Start the potatoes in a large stock pot filled with cold water and season generously with kosher salt until it tastes like a mild broth. Slowly bring the water to a gentle simmer—never a boil—to avoid breaking up the potatoes. Simmer until the potatoes are fork-tender, then drain and set aside.

Meanwhile, melt the butter into the cream over low heat, ensuring it doesn’t come to a boil. Return the cooked potatoes to the pot and gently smash them, leaving plenty of texture. Pour the warm cream mixture over the potatoes and stir just to combine. Add horseradish, grated smoked cheddar, and salt. Fold everything together until the cheese melts and the flavors are fully integrated. Adjust seasoning to taste.

In a large rondeau, combine the onions, oil, and salt. Cover and cook over low heat for about 20 minutes to allow the onions to sweat. Remove the lid and continue cooking on low, stirring occasionally for 2 to 3 hours. The onions are done when they are a deep golden brown and have taken on a rich, jam-like consistency.

Garnish or serve with onions, cheddar, and horseradish as desired.