Whisk the soy sauce and cayenne pepper in a small bowl until the cayenne pepper is dissolved; set aside.
Heat the vegetable oil in a large skillet over medium-high heat; cook the potatoes in the hot oil until golden brown, about 5 minutes. Stir in the onions, bell pepper, and sesame seeds; cook 1 minute more. Pour the soy sauce mixture over the potatoes; cook and stir until the liquid is completely absorbed, 1 to 2 minutes.