Classic comfort meets easy elegance with this steak Marsala. Our take on this Italian restaurant staple pairs a tender sirloin steak with a rich reduction of fortified Marsala cooking wine and cremini mushrooms. Served next to Parmesan mashed potatoes, your chef skills will be on full display when you set this dish down on the table.
Peel and cut potato into 1" dice. Bring a medium pot with potato and enough lightly salted cold water to cover to a boil. Reduce to a simmer and cook until tender, 18-20 minutes. While potato cooks, trim and thinly slice green onions on an angle, keeping white and green portions separate. Quarter mushrooms. Pat steaks dry, and season both sides with a pinch of salt and ¼ tsp. pepper.
Cook the Steaks
Heat a medium pan over medium-high heat. Add 2 tsp. olive oil and steaks to hot pan and cook until browned, 2-3 minutes per side. Transfer to prepared baking sheet and roast until steaks reach a minimum internal temperature of 145 degrees, 5-7 minutes. Transfer steaks to cutting board and cover loosely with foil. Reserve pan; no need to wipe clean.
Mash the Potato
Drain potato in a colander and return to pot with half the evaporated milk (reserve remaining for sauce), Parmesan, and half the green portions of green onions (reserve remaining for garnish). Mash, adding remaining evaporated milk 1 Tbsp. at a time until desired consistency is reached, reserving at least 2 Tbsp. for sauce. No additional evaporated milk may be needed. Season with ¼ tsp. salt. Set aside and keep warm.
Start the Sauce
Return pan used to cook steaks to medium-high heat. Add 1 tsp. olive oil, mushrooms, and ¼ tsp. salt to hot pan. Stir occasionally until mushrooms begin to caramelize, 5-8 minutes.
Finish the Sauce
Add white portions of green onions and Marsala wine to pan and cook until liquid is reduced by half, 1-2 minutes. Add beef demi-glace, remaining evaporated milk, any accumulated juices from resting steaks, and ½ cup water to pan and simmer until sauce has thickened to consistency of a light gravy, 2-4 minutes.
Finish the Dish
Use a slotted spoon to separate mushrooms from sauce in pan and place mushrooms on a plate. Spoon sauce onto plate and set steak on sauce. Serve Parmesan mashed potatoes next to steak and garnish mash and mushrooms with remaining green portions of green onions.