Preheat your oven to 400°F and slice the mini potatoes into halves or quarters so they are bite-sized.
Toss the potatoes on a parchment-lined baking sheet with 0.5 tbsp of olive oil, garlic powder, and a pinch of the salt and pepper.
Roast the potatoes for 20-25 minutes, flipping halfway through, until they are golden brown and fork-tender.
While potatoes roast, season the sirloin steak with the remaining salt and pepper.
Heat a cast-iron skillet over medium-high heat and sear the steak for 3-5 minutes per side, depending on thickness, until it reaches your preferred doneness.
Remove the steak from the pan and let it rest on a cutting board for at least 5 minutes before slicing into thin strips.
In a small jar or bowl, whisk together the remaining 0.5 tbsp olive oil, balsamic vinegar, and Dijon mustard to create the dressing.
Place the spring mix in a large serving bowl, top with the warm roasted potatoes and steak strips, then drizzle with the balsamic vinaigrette and serve immediately.