To make dressing, combine fish sauce, lime juice, water, sugar, garlic, shallot and ginger in a small bowl and stir until sugar dissolves. Put steak in a shallow bowl, pour in 1/4 cup of dressing and marinate 15 to 20 minutes.
Cook noodles according to package directions.
Preheat grill for high heat. Lift steak from marinade, pat dry and grill for 4 to 5 minutes per side for medium-rare. Let rest for 10 minutes before slicing it into very thin strips.
To serve, tear lettuce into the bottom of 4 bowls. Add noodles and add carrots, cucumber, cilantro and mint. Top with steak and dressing. Add a splash of Sriricha, if using.