How to Make It
Light a grill. Rub the flank steak with the soy sauce. Cover and marinate at room temperature while you make the dressing.
In a small bowl, whisk the lime juice with the fish sauce and brown sugar until the sugar dissolves. Whisk in the tomato paste, curry paste, garlic and shallot.
Pat the steak dry with paper towels and rub each side with the vegetable oil. Grill the steak over high heat, rotating it 90 degrees halfway through, until browned with crosshatch marks, about 3 minutes. Flip the steak and grill until an instant-read thermometer inserted in the thickest part registers 125° for medium rare, about 4 minutes. Transfer the steak to a work surface and let stand for 3 minutes. Thinly slice the steak crosswise against the grain.
Add the cilantro to the dressing. In a medium bowl, whisk 2 tablespoons of the dressing with the olive oil; add the greens and toss well. Mound the salad on plates. In the same bowl, toss the steak with another 2 tablespoons of the dressing. Arrange the steak on the greens and serve, passing the remaining dressing at the table.