Wash and dry all produce. Cut potatoes into 1/2-inch cubes. Trim, then finely chop scallions. Trim and discard bottom root ends from bok choy, then separate stalks. Mince or grate garlic. Peel, then mince or grate ginger until you have 1 TBSP. Finely chop cilantro. Cut lime into wedges.

Stir together hoisin, soy sauce, and a drizzle of oil in a medium bowl. Stir to combine. Add steak and toss to coat. Set aside to marinate. Place potatoes in a medium pot along with enough salted water to cover by 1 inch. Bring to a boil and cook until tender, about 12 minutes.

Reserve 1/2 cup potato cooking water, then drain potatoes. Melt 2 TBSP butter in same pot over low heat. Add half the garlic and 1/3 of the scallions. Cook, tossing, until fragrant, 1-2 minutes. Return potatoes to pot and mash until smooth. Stir in 1/4 cup reserved water (add more if potatoes are stiff).

While potatoes cook, in another medium bowl, combine cilantro, sesame oil, remaining 2/3 of the scallions, and as much of the minced ginger and remaining garlic as you like (use both to taste). Stir in 2 TBSP olive oil, 1 tsp sugar, and a squeeze or two of lime juice (to taste). Stir in half the sesame seeds. Season with salt and pepper.

Heat a large drizzle of oil in a medium pan over medium-high heat. Remove steak from marinade, letting any excess drip off, then add steak to pan. Cook to desired doneness, 3-5 minutes per side. Remove from pan and set aside to rest. Rinse out pan. Add another drizzle of oil to pan over medium-high heat. Add bok choy and any remaining ginger. Season with salt and pepper. Cook until tender, 2-4 minutes.

Season potatoes with salt and pepper, then divide between plates. Add bok choy to the side. Slice steak against the grain and arrange on top of potatoes. Spoon scallion pesto over steak, then sprinkle with remaining sesame seeds. Drizzle with sriracha (to taste). Serve with remaining lime wedges on the side.