Place 2 TBSP butter (4 TBSP for 4 servings) in a medium bowl; bring to room temperature. Wash and dry produce. Trim and thinly slice scallions, separating whites from greens. Peel and finely chop garlic. In a small bowl, combine mustard, stock concentrate, ponzu, cornstarch, ⅓ cup water, 2 TBSP ketchup, and 1 tsp brown sugar (if you don’t have brown sugar, simply swap in the same amount of white sugar!). (For 4, use ½ cup water, 4 TBSP ketchup, and 2 tsp brown sugar.)

Meanwhile, poke potatoes all over with a fork; rub with oil and season with salt and pepper. Place on a microwave-safe plate. Microwave, carefully flipping with tongs halfway through, until tender, 9-13 minutes. Set aside to cool slightly. (If potatoes are still firm, continue to microwave in 1-minute intervals until easily pierced by a fork.)

Meanwhile, heat a drizzle of oil in a large pan over medium-high heat. Add beef and a big pinch of salt. Cook, breaking up meat into pieces, until browned and cooked through, 4-5 minutes. (If there’s excess grease in your pan, carefully pour it out.) Add scallion whites and garlic; cook, stirring, until fragrant, 30- 60 seconds. Add sauce mixture and bring to a boil. Cook until thickened, 2-3 minutes. Stir in 1 TBSP butter (2 TBSP for 4 servings) until melted. Season with salt and pepper.

Carefully halve potatoes lengthwise; fluff insides with a fork, then scoop into bowl with softened butter, leaving skins intact. Add sour cream and half the scallion greens to bowl, mashing with a fork to combine. Season generously with salt and pepper. Stuff potato filling back into skins.

Divide potatoes between plates and top with saucy beef and remaining scallion greens.