In a bowl, whisk together lime juice, fish sauce, oil, garlic, sugar and red pepper flakes. Set aside.
Heat a grill or grill pan to medium-high. Spray or brush lightly with oil. Season steak on both sides with salt. Cook on medium-high 3 to 5 minutes per side, until medium-rare. Let rest 10 minutes to cool, then slice thinly against the grain. (Alternately, steak can be cooked ahead of time, refrigerated, then sliced.)
Roughly chop cilantro and mint, then gently toss with sliced steak, dressing, romaine, tomatoes, sweet pepper, cucumber and shallots. Serve immediately.