Preheat the grill to medium-high heat.

Season the sirloin steak with salt and pepper.

Grill the steak for about 4-5 minutes on each side for medium-rare, or until desired doneness.

Remove from grill and let rest for 5 minutes before slicing.

In a large mixing bowl, combine the mixed salad greens, cherry tomatoes, cucumber, avocado, and red onion.

In a separate bowl, whisk together the olive oil, balsamic vinegar, Dijon mustard, minced garlic, salt, and pepper to make the vinaigrette.

Slice the grilled steak against the grain into thin strips.

Top the salad with the sliced steak and drizzle with the vinaigrette. Toss gently to combine.