Adjust rack to top position (top and bottom positions for 4 servings) and preheat oven to 425 degrees. Wash and dry produce. Dice potatoes into ½-inch pieces. Trim, peel, and cut carrots on a diagonal into ½-inch-thick pieces. Strip half the thyme leaves from stems (all for 4); roughly chop leaves. Peel and mince or grate garlic. Trim and thinly slice scallions, separating whites from greens.
Toss potatoes on one side of a baking sheet with a drizzle of olive oil; season generously with salt and pepper. Toss carrots on empty side with a drizzle of olive oil; season with salt and pepper. (For 4 servings, divide between 2 sheets; roast potatoes on top rack and carrots on bottom rack.) Roast on top rack until golden brown and crispy, 20-25 minutes.
Meanwhile, pat steak* dry with paper towels; season generously all over with salt and pepper. Heat a drizzle of oil in a large pan over medium-high heat. Add steak and cook to desired doneness, 4-7 minutes per side. Turn off heat; transfer to a cutting board to rest. Wipe out pan.
Heat a drizzle of olive oil and 1 TBSP butter (2 TBSP for 4 servings) in same pan over medium-high heat. Add chopped thyme, garlic, and scallion whites; cook until fragrant, 30 seconds.
Stir stock concentrate and ¼ cup water (⅓ cup for 4 servings) into pan with aromatics. Bring to a simmer, then stir in any resting juices from steak; cook until slightly thickened, 1-2 minutes. Remove pan from heat. Stir in sour cream until smooth. Season with salt and pepper.
Slice steak against the grain. Divide steak, potatoes, and carrots between plates. Spoon sauce over steak. Garnish with scallion greens and serve.
Notes:
I used NY Strip steak for this, and added mushrooms to the sauce.