Season the steak with salt and pepper. Heat the oil in a griddle or saute pan until smoking hot and sear the steak for 1 to 2 minutes on each side until browned but still rare in the middle. Set aside to rest.
Mix the sweet chile sauce with the lime juice in a serving bowl. Add the cucumber, carrots, mango and baby gem lettuces, along with the herbs, and toss through to combine.