Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Trim off bottom 2 inches from asparagus. Dice potatoes into 1/2-inch cubes (no need to peel). Remove tarragon leaves from stems; mince leaves until you have 2 tsp.

Place potatoes in a medium, lidded pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender when pierced with a fork, about 15 minutes. Reserve 1/2 cup cooking water, then drain and return potatoes to pot. Add half the garlic oil (add more to taste), half the sour cream, 1 TBSP butter, and 1/4 cup reserved cooking water. Mash with a potato masher or fork, adding more cooking water if necessary. Season with salt.

While potatoes cook, toss asparagus on a baking sheet with a drizzle of olive oil, 1 tsp minced tarragon (save the rest for later), salt, and pepper. Roast until tender, 8-12 minutes.

While asparagus roasts, pat steak very dry with paper towels; season all over with salt and pepper. Heat a drizzle of oil in a large pan over medium-high heat. Once oil is hot, add steak and cook to desired doneness, 2-4 minutes per side, depending on thickness of meat. (TIP: If steak has a layer of fat at the top, use tongs to flip meat and sear until fat is rendered and crispy.) Turn off heat; transfer to a cutting board to rest. Wipe out pan.

Add stock concentrate, mustard, and 1/4 cup water to same pan. Bring to a simmer over medium heat. Cook, whisking, until thickened, about 2 minutes. Turn off heat; stir in remaining minced tarragon, remaining sour cream, and 1 TBSP butter until melted. Season with salt and pepper.

Slice steak against the grain. Divide asparagus, potatoes, and steak between plates. Drizzle steak with sauce. Garnish with as many remaining tarragon leaves as you like.