Heat olive oil in a skillet, cook sliced beef until browned and cooked to desired doneness.

Season beef with salt and pepper, then let it rest briefly.

In a large bowl, combine mixed greens, cherry tomatoes, cucumber, and red onion.

In a small bowl, whisk lemon juice and Dijon mustard to create dressing.

Add beef and avocado to salad, drizzle dressing, then toss gently.

Serve immediately for best flavor and texture.