Season the steak with salt and pepper. Finely grind 2 teaspoons cumin seeds in a spice grinder (or grind them with the bottom of a skillet). Toss the steak with the ground cumin, soy sauce, sesame oil and sugar in a large bowl; let sit 10 minutes. Meanwhile, chop the bell pepper, slice the chile into very thin rounds and cut the scallions into 1-inch pieces.
Preheat a large skillet or wok over high heat until very hot. Add 1 tablespoon vegetable oil, then add the bell pepper, chile, scallions and a pinch of salt. Cook, undisturbed, until the vegetables are charred in spots, about 2 minutes. Toss a few times and continue cooking until crisp-tender, about 1 more minute; remove to a large plate.
Add the remaining 1 teaspoon vegetable oil to the skillet, then add the steak, spreading it out in an even layer. Sprinkle with the remaining 2 teaspoons whole cumin seeds and cook, undisturbed, until the steak is crisp around the edges and charred in spots, 3 to 5 minutes. Toss a few times and continue cooking until just cooked through, 2 to 3 minutes. Drain off any fat from the skillet, then return the vegetables to the pan along with half the cilantro; toss. Serve over the rice and top with the remaining cilantro.