Adjust rack to top position and preheat oven to 425 degrees. Bring a medium pot of salted water to a boil. Wash and dry all produce. Trim and thinly slice scallions, separating whites from greens. Trim green beans, if necessary.

Once water is boiling, add couscous to pot. Cook until al dente, 6-8 minutes. Drain, then place empty pot over medium heat. Add truffle butter and scallion whites; cook, stirring, until softened, 1-2 minutes. Stir in cooked couscous until evenly coated. Turn off heat; cover until ready to serve.

Meanwhile, pat beef dry with paper towels; season with salt and pepper. Sprinkle with half the Steak Spice (use all for 4 servings), pressing to adhere. Heat a large drizzle of oil in a large pan over medium-high heat. Add beef and cook until a crust forms, 2-3 minutes per side (it’ll finish cooking in the next step). Turn off heat; transfer beef to one side of a baking sheet. Wipe out pan.

Toss green beans on opposite side of sheet from beef with a drizzle of olive oil, salt, and pepper. Roast on top rack until green beans are browned and tender and beef reaches desired doneness, 10-12 minutes. (TIP: Beef may finish before green beans.) Remove from oven; let beef rest for 5 minutes.

Meanwhile, heat pan used for beef over medium-high heat. Stir in stock concentrate and 1/4 cup water (1/3 cup for 4 servings). Simmer until slightly thickened, 1-2 minutes. Reduce heat to low and whisk in mustard. Turn off heat; stir in sour cream and any resting juices from beef. Season with salt and pepper. TIP: If sauce gets too thick, stir in hot water 1 TBSP at a time until it reaches a drizzling consistency.

Fluff couscous with a fork; stir in Parmesan and half the scallion greens. Season with salt and pepper. Divide couscous, green beans, and beef between plates. Spoon sauce over beef. Sprinkle with remaining scallion greens and serve.