For the steak: Whisk the fish sauce, soy sauce, brown sugar and pepper in a small bowl until the sugar is dissolved. Prick the steak all over with a fork and put in a resealable plastic bag. Add the fish sauce mixture, seal the bag and massage the marinade into the meat for a few minutes. Let marinate 15 minutes.

Meanwhile, for the dressing: Whisk the lime juice, fish sauce, brown sugar, chile and garlic in a small bowl until the sugar is dissolved.

Heat a grill pan over medium-high heat. Remove the steak from the marinade; drizzle both sides with the vegetable oil. When the pan is hot, grill the steak to desired doneness, 3 to 5 minutes per side for medium rare. Remove to a cutting board and let rest 10 minutes, then slice very thinly against the grain.

For the salad: Combine the tomatoes, romaine, cucumber, red onion, cilantro, mint and peanuts in a large bowl. Toss the salad with half of the dressing, then mound on a platter or individual plates. Toss the sliced steak and its juices with the remaining dressing in the same bowl. Top the salad with the steak and any remaining dressing from the bowl.