Season the sirloin steak with salt, black pepper, and any preferred steak seasoning (optional). Sear or grill the steak over medium-high heat until medium doneness, about 3–5 minutes per side depending on thickness.
Transfer the cooked steak to a cutting board and let it rest for 5 minutes, then slice thinly against the grain.
Place the mixed baby greens in a large salad bowl.
Add the avocado chunks, diced mango, and fresh mozzarella to the greens.
In a small bowl or jar, whisk together the olive oil, balsamic vinegar, honey, salt, and black pepper until the dressing is smooth and combined.
Drizzle the dressing over the salad, top with the sliced steak, and toss gently if desired. Serve immediately.