Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Dice potatoes into 1/2-inch pieces. Trim and halve Brussels sprouts. Halve, peel, and mince shallot until you have 2 TBSP (3 TBSP for 4 servings). Strip rosemary leaves from stems; mince leaves until you have 1 tsp (2 tsp for 4). Mince or grate garlic.

Place potatoes in a medium pot with enough salted water to cover by 2 inches. Bring to a boil; cook until tender, 15-20 minutes. Reserve 1 cup potato cooking liquid, then drain. Melt 2 TBSP butter (4 TBSP for 4) in same pot over medium heat. Add garlic; cook 30-60 seconds. Return potatoes to pot and mash with a splash of reserved potato cooking liquid. Stir in sour cream. Season with salt and pepper.

Meanwhile, heat a drizzle of olive oil in a medium, preferably nonstick, pan over medium-high heat. Add panko; season with salt and pepper. Cook, stirring often, until golden brown, 3-5 minutes. Turn off heat; transfer panko to a medium bowl. Wipe out pan.

Toss Brussels sprouts on a baking sheet with a large drizzle of olive oil, salt, and pepper. Arrange cut sides down and roast until browned and crispy, 15-20 minutes. Add to bowl with panko and toss to combine. Season with salt, pepper, and half the minced rosemary (save the rest for later).

Meanwhile, pat beef dry with paper towels; season all over with salt and pepper. Heat a large drizzle of oil in pan used for panko over medium-high heat. Add beef and cook to desired doneness, 3-6 minutes per side. Turn off heat; remove from pan and set aside to rest for 5 minutes. Wipe out pan.

Melt 1 TBSP butter in same pan over medium heat. Add minced shallot; cook until fragrant, 1-2 minutes. Add stock concentrate, jam, vinegar, 2 TBSP water (4 TBSP for 4 servings), and remaining minced rosemary to taste. Cook until slightly thickened, 1-2 minutes. Stir in any resting juices from beef; season with salt and pepper. Divide potatoes, Brussels sprouts, and beef between plates. Top beef with sauce.