Wash and dry all produce. Trim, then thinly slice scallions, separating greens and whites. Set aside. Cut potatoes into 1/2-inch cubes and place in a large pot with enough salted water to cover by 2 inches. Bring to a boil over high heat. Cook until tender, about 15 minutes. Drain well.

Return empty pot to stove over low heat. Add garlic herb butter and half the scallion whites. Let butter melt and scallion whites soften, 1-2 minutes. Return potatoes to pot and mash until smooth. Stir in sour cream and half the scallion greens. Season with salt and pepper. Keep covered on stove over low heat until ready to serve.

Meanwhile, heat a large drizzle of oil in a large pan over medium-high heat. Pat steak dry with a paper towel; season with plenty of salt and pepper. Add to pan and cook to desired doneness, 4-7 minutes per side. Remove from pan and set aside on a plate to rest.

In a second large pan, heat sesame oil over medium-high heat. Add snow peas and edamame; season with salt and pepper. Cook, tossing, until snow peas are bright green and edamame is lightly charred, 3-4 minutes. Toss in half the sesame seeds. Season with salt and pepper.

Wash out any blackened bits in pan used for steak; dry well. Heat a drizzle of oil in same pan over medium-high heat. Add remaining scallion whites and cook until softened, 1-2 minutes. Stir in soy sauce, demi-glace, hoisin sauce, and 1/4 cup water. Bring to a simmer and let reduce slightly, 1-2 minutes. Reduce heat to low, then stir in 2 TBSP plain butter. Stir in any juices released by steak, then season with salt and pepper.

Divide potatoes, snow peas, and edamame between plates. Slice steak against the grain and arrange next to potatoes. Spoon sauce over steak, then garnish with remaining scallion greens and sesame seeds. Drizzle with sriracha to taste.