Begin by marinating the beef with salt, pepper and garlic (if you have time, leave it overnight for best results!). When you’re ready to cook, grill beef on medium high for approximately 6 minutes on each side. Let cool slightly, and then slice thinly.
Then whisk together lime, soy sauce, fish sauce, sugar and pepper flakes in a large bowl. Add the vegetables and beef, and mix to coat thoroughly. Refrigerate for 1 hour, stirring every 15 minutes.