Preheat a grill or frying pan over medium-high heat. Season the beef with salt and pepper on both sides.
Grill the beef for 3-4 minutes on each side or until it reaches your desired doneness. Remove from heat and let it rest for 5 minutes before slicing thinly.
In a large bowl, combine the salad greens, cucumber, tomato, cilantro, and mint.
In a small bowl, mix together the fish sauce, lime juice, brown sugar, soy sauce, and sliced chili to make the dressing.
Add the sliced beef to the vegetable mixture and pour over the dressing. Toss gently to combine all ingredients.
Serve immediately and enjoy your Thai Beef Salad.