Heat oven to 425°F. On a large rimmed baking sheet, toss the potatoes and red pepper with 2 Tbsp oil, 1 Tbsp paprika and 1/2 tsp each salt & pepper. Arrange in an even layer and roast, turning once or twice, until the potatoes are golden brown and tender, 20 to 25 minutes.

Meanwhile, heat the remaining 2 tsp oil in a large skillet over medium-high heat. Season the flank steak with 1/2 tsp each salt & pepper and cook to desired doneness, 5 to 7 minutes per side for medium-rare. Transfer to a board and let rest for 5 minutes before slicing.

In a small bowl, whisk together the mayonnaise, lemon juice, mustard and remaining 1 tsp paprika.

Toss the potatoes and red pepper with the scallions and serve with the steak and dipping sauce.