Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Peel the sweet potato and cut it into small, uniform cubes.
Toss the sweet potato cubes with half of the olive oil and a pinch of salt and pepper, then spread them on the baking sheet.
Roast the sweet potatoes for 20-25 minutes until they are tender and slightly caramelized on the edges.
Season the sirloin steak with garlic powder, salt, and pepper on both sides.
Heat a grill pan or outdoor grill over medium-high heat and lightly grease with the remaining olive oil.
Grill the steak for 4-5 minutes per side for medium-rare, or until it reaches your preferred level of doneness.
Remove the steak from the heat and let it rest for 5 minutes before slicing it against the grain into thin strips.
In a large bowl, arrange the mixed greens and top with the roasted sweet potatoes and grilled beef strips.
Drizzle the balsamic vinegar over the salad and serve while the beef and potatoes are still warm.