Quarter mushrooms.

Halve and peel onion. Slice halves into thin strips.

Peel and cut potatoes into large chunks.

Pat steaks dry, and season both sides with 1/4 tsp. salt. Season one side with coarse black pepper.

Place mushrooms on one half of prepared baking sheet and toss with 2 tsp. olive oil and a pinch of salt and pepper. Add onion to empty half and toss with 1 tsp. olive oil and a pinch of salt.

Spread mushrooms and onion into even layers on their sides (some overlap is ok). Roast in hot oven until both are browned and softened, 18-20 minutes.

While mushrooms and onion roast, make mashed potatoes.

Bring a medium pot with potatoes covered by lightly salted water to a boil. Cook until tender, 14-18 minutes.

Reserve 1/2 cup potato cooking water. Drain potatoes in a colander and return to pot.

Add sour cream, 2/3 the butter (reserve remaining for sauce), and 1/4 tsp. salt and mash until smooth. If too dry, add reserved potato cooking water, 1 Tbsp. at a time, until desired consistency is reached. Cover and set aside.

While potatoes boil, cook steaks.

Place a medium non-stick pan over medium heat and add 1 tsp. olive oil. Place steaks in hot pan, peppered-side down. Cook until browned, pepper is aromatic, and steak reaches a minimum internal temperature of 145 degrees, 5-7 minutes per side.

Remove steaks to a plate and tent with foil. Wipe pan clean and reserve.

Return pan used to cook steaks to medium-high heat. Add demi-glace and 1/4 cup water to hot pan. Stir, then bring to a boil.

Once boiling, cook until thickened slightly, 1-2 minutes.

Remove from burner and swirl in remaining butter until combined.

Plate dish as pictured on front of card, placing steak on sauce and topping mashed potatoes with roasted onions. Bon appétit!