Wash and dry all produce. Adjust rack to upper position and preheat oven to 425 degrees. Roughly chop hazelnuts. Peel potatoes, then cut into 1/2 inch cubes. Halve carrots lengthwise through stem ends. Mince or grate garlic. Pick parsley leaves from stems and finely chop. Pat steak dry with paper towel.

Toss carrots with a drizzle of oil, 3/4 tsp paprika (we’ll use the rest later), and plenty of salt and pepper on a baking sheet. Roast in oven until tender, 20-25 minutes. Meanwhile, place potatoes in a medium pot with enough salted water to cover by 1 inch. Bring to a boil. Cook until tender, about 15 minutes.

Heat a large pan over medium-high heat. Add hazelnuts and toast, tossing frequently, until golden brown and fragrant, 3-5 minutes. (TIP: Remove pan from heat for a few moments if nuts seem like they may burn.) Remove from pan and set aside.

Heat a drizzle of oil in same pan over medium-high heat. Season steak all over with salt, pepper, and steak spice. Cook in pan to desired doneness, 4-7 minutes per side. Remove from pan and set aside. Meanwhile, microwave 1 TBSP butter in a small bowl until just softened, about 10 seconds. Add 1/2 tsp garlic, 1 tsp parsley, mustard, and remaining paprika to bowl; mash to combine. Let chill in fridge.

Once potatoes are tender, reserve 1/4 cup cooking water, then drain. Heat a drizzle of oil in same pot over medium heat. Add remaining garlic and cook until fragrant, 30 seconds to 1 minute. Reduce heat to lowest setting, then add potatoes. Mash until smooth. Mix in sour cream and half the cheese. Add a few splashes of reserved water, if needed, to give potatoes a creamy consistency. Season generously with salt and pepper.

Toss carrots with hazelnuts on sheet. Slice steak against the grain. Divide potatoes between plates and sprinkle with remaining cheese. Add steak and carrots to plates. Dollop steak with chilled butter and garnish with remaining parsley.