Place peeled, cubed potatoes in a large pot and cover with cold water. Add a pinch of salt.

Bring to a boil, then simmer 15–20 minutes until fork-tender.

Drain well and return to the pot over low heat for 30 seconds to evaporate excess moisture.

Mash with 4 tablespoons unsalted butter, 1/2 cup milk, and 1/4 cup heavy cream. Season with salt and pepper. Keep warm.

Pat steaks dry and season both sides with salt and pepper. Let rest at room temperature for 20–30 minutes.

Heat a heavy skillet (preferably cast iron) over medium-high heat. Add 2 tablespoons olive oil or butter until shimmering.

For 1-inch steaks, sear 3–4 minutes per side for medium-rare. Adjust time for thickness.

Use an instant-read thermometer: remove steaks at about 125–130°F for medium-rare, 135°F for medium.

Transfer steaks to a plate and tent loosely with foil; rest for 5–10 minutes.

In the same skillet with steak drippings, reduce heat to medium and add 2 tablespoons butter.

Add sliced mushrooms and sauté until golden, about 5–7 minutes. Season lightly.

Stir in minced garlic for 30 seconds until fragrant.

Pour in 1 cup beef broth, scraping browned bits from the pan.

Stir in 1/2 cup heavy cream and simmer for 2–3 minutes.

Whisk in the cornstarch slurry gradually until the gravy reaches your desired thickness. Taste and adjust seasoning.

Spoon mashed potatoes onto plates or a family platter.

Slice steaks against the grain and place on or beside the mash.

Spoon mushroom gravy generously over steak and potatoes. Garnish with chopped parsley if desired.