Peel potato and cut into 1" cubes. Bring a small pot with potato cubes, 1 tsp. salt, and enough water to cover to a boil. Reduce to a simmer and cook until tender, 15-18 minutes. While potatoes simmer, mince chives. Trim bottom ends from broccolini. Peel and mince shallot. Pat steaks dry.

Drain potato in a colander and return to pot. Add sour cream, liquid egg, half the butter, and chives to pot and mash until smooth. Season with 1/4 tsp. salt and a pinch of pepper. Transfer to a small casserole dish or two oven-safe ramekins and place on foil-only prepared baking sheet. Use fork tines to pull potatoes into small peaks, forming decorative ridges. Bake until potato ridges turn golden brown, 15-22 minutes. While potatoes cook, sear steaks.

Place a medium non-stick pan over high heat. Season steaks on both sides with 1/4 tsp. salt and a pinch of pepper. Add 1 tsp. olive oil and steaks to hot pan and cook until browned, 2-3 minutes per side. Transfer steaks to one side of sprayed prepared baking sheet and set aside. Reserve pan; no need to wipe clean.

Place broccolini on same baking sheet as steaks and toss with 1 tsp. olive oil and a pinch of salt and pepper. Roast until steaks reach a minimum internal temperature of 145 degrees, 8-14 minutes. Tent steaks and broccolini with foil and rest steaks at least 5 minutes.

Return pan used to sear steaks to medium-high heat. Add 1 tsp. olive oil and 2 Tbsp. shallot to pan and cook 30 seconds. Add red wine and reduce to a syrupy consistency. Add demi-glace and ¼ cup water and cook until slightly thickened, 1 minute. Remove from burner and swirl in remaining butter. Strain sauce if desired.

Arrange roasted broccolini and steak on a plate. Spoon sauce onto plate and serve duchess potatoes on the side. Garnish potatoes with remaining chives.