Pat the salmon dry. Heat a medium skillet over medium-high heat and cook the salmon, 4 to 6 minutes per side, until opaque and cooked through. Transfer to a plate, let rest 1 minute, then flake into small chunks with a fork. Wipe the skillet clean and set the salmon aside.
Reserve 1/2 teaspoon of the sesame oil for finishing. Heat the remaining 1/2 teaspoon sesame oil in a medium nonstick skillet over medium-high heat.
Add the scallion whites and cook, stirring, until fragrant, about 1 minute.
Add the cooked cold rice in an even layer. Cook without stirring for 2 to 3 minutes, until the bottom becomes slightly crispy.
Add the cauliflower rice (fresh or frozen) and continue to cook, stirring occasionally, 2 to 3 minutes, until the cauliflower rice is heated and combined with the rice.
Push the rice and cauliflower mixture to one side of the skillet to clear the other side. Pour the beaten egg into the cleared side and cook, stirring constantly, 30 to 60 seconds, until the egg is cooked through and scrambled.
Mix the cooked egg into the rice and cauliflower until evenly combined. Stir in the soy sauce and the reserved 1/2 teaspoon sesame oil.
Gently fold in the flaked salmon. Remove from heat, sprinkle with the scallion greens, and serve immediately with Sriracha or chile-garlic sauce if desired.