Pat the chicken dry and toss with 1/4 teaspoon salt. Heat a large nonstick skillet or wok over medium-high heat with 1/2 Tablespoon oil. Add the chicken in a single layer and let it sear, stirring occasionally, until cooked through and lightly browned, about 6–8 minutes. Transfer chicken to a plate and set aside.
Lower the heat to medium and add 1/2 Tablespoon butter to the empty pan. Sauté the onion until translucent, about 3 minutes, then add the garlic and ginger and cook another 45 seconds until fragrant. Stir in the frozen vegetables and sprinkle with 1/4 teaspoon salt; cook, stirring, until just tender, 3–4 minutes.
Push the vegetables to one side of the skillet and melt a small pat of butter or a little oil in the cleared space. Pour in the beaten eggs and gently scramble until mostly set but still soft, 30–60 seconds. Fold the eggs together with the veggies and move them off to the side with the chicken.
Increase heat to medium-high and add the remaining 1 Tablespoon butter. Add the chilled rice, breaking up any clumps, and spread it into an even layer. Let it sit without stirring for 3–4 minutes so the bottom can get golden and crisp. Stir and repeat once so you get pockets of crunchy rice.
Return the chicken, egg and vegetable mixture to the pan with the rice. Pour the soy sauce evenly around the pan and toss everything together, stirring and flipping the rice until the sauce is absorbed and the ingredients are heated through, about 1–2 minutes.
Remove the pan from the heat and stir in the toasted sesame oil and sliced green onions. Taste and adjust with a pinch more salt if needed. Let the fried rice rest in the pan for 1–2 minutes — this helps the flavors meld before serving.
Spoon into bowls, top with extra scallions and a drizzle of sriracha or a sprinkle of red pepper flakes if you like heat. Serve hot.