Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Dice the red potatoes into small, even cubes and toss them with half of the olive oil, garlic powder, and a pinch of salt.
Spread the potatoes on the baking sheet and roast for 20-25 minutes until they are golden and crisp.
Season the flank steak with salt and cracked black pepper on both sides.
Heat a grill pan or cast-iron skillet over medium-high heat and cook the steak for 4-5 minutes per side for medium-rare doneness.
Remove the steak from the heat and let it rest for at least 5 minutes before slicing thinly against the grain.
In a small jar or bowl, whisk together the remaining olive oil, balsamic vinegar, and Dijon mustard to create the vinaigrette.
Assemble the bowl by placing the mixed greens at the bottom, then top with the roasted potatoes and sliced steak, drizzling the dressing over everything just before serving.