Season steaks generously on all sides with kosher salt, all-purpose seasoning, and a touch of chili powder. Let rest at room temperature for at least 30 minutes. Pat completely dry before cooking — moisture is the enemy of a good sear.

Cube potatoes into uniform pieces and add to a pot of cold salted water. Bring to a boil and cook until fork-tender, about 15–18 minutes.

Warm heavy cream with the rosemary sprig in a small saucepan over medium-low heat. Let steep 5–8 minutes until fragrant, then strain and set aside.

Dice the shallot, mince the garlic, and slice the mushrooms. Have everything prepped before the heat goes up — this sauce moves fast.

Get a heavy stainless or cast iron skillet ripping hot over high heat. Add a high smoke-point oil and sear steaks 2–3 minutes per side for medium-rare. Don't touch them — let the crust develop. Pull steaks and tent loosely with foil.

Reduce heat to medium. Add butter to the same pan and sauté mushrooms in a single layer until browned and caramelized. Add shallots and garlic, cook another 2 minutes.

Stir in tomato paste and cook 1–2 minutes until it darkens slightly — this caramelization adds serious depth that most recipes skip.

Carefully add cognac and let reduce by half, scraping up all the fond from the bottom of the pan. The alcohol cooks off; what's left is concentrated flavor.

Add beef base, Worcestershire sauce, soy sauce, and Dijon mustard. Stir to combine. Pour in heavy cream and simmer 3–5 minutes until the sauce thickens and coats the back of a spoon.

Drain potatoes and let steam dry 1–2 minutes in the hot pot. Mash with butter, rosemary-infused cream, sour cream, garlic, and seasoning. Spoon onto plates, top with steak, and ladle sauce over everything. Garnish with fresh parsley.