In a bowl, toss the shrimp with cayenne, chili powder, paprika, garlic powder, kosher salt, and freshly ground black pepper until evenly coated.
Heat a wok or large skillet over medium-high heat and lightly spray with oil. Add the beaten egg whites and whole egg; cook, stirring, until just set, about 1–2 minutes. Transfer the scrambled eggs to a plate and set aside.
Increase heat to high. Add the sesame oil, then sauté the chopped onion, scallion whites, diced garlic, and crushed red pepper flakes for 1–2 minutes until fragrant and the onion is softened.
Add the seasoned shrimp and cook, stirring, until opaque and cooked through, about 2–3 minutes.
Add the leftover brown rice to the wok and let it sit undisturbed for 2–3 minutes to brown on the bottom, then stir to break up any clumps.
Return the cooked eggs to the pan and add the soy sauce and fish sauce. Stir and cook everything together for about 2 minutes, mixing well so the sauces coat the rice and shrimp.
Stir in the scallion greens, adjust seasoning to taste, and serve immediately.
