Mix the fish sauce and thinly sliced chili in a small bowl and set aside.
Heat the vegetable oil in a very hot large skillet or wok over medium-high heat.
Crack the eggs into the skillet and scramble quickly until just set, then push them to the side.
Add the diced chicken to the skillet and cook until browned and cooked through, about 4–5 minutes.
Add the chopped onion, chopped tomato, and frozen peas and carrots; stir and cook until the vegetables are heated through and the onion softens.
Add the cold cooked rice, sugar, black pepper, soy sauce, and the reserved fish sauce–chili mixture; stir constantly until the rice is heated, dry, and slightly glossy.
Taste and adjust seasoning if desired, then remove from heat.
Garnish with Thai basil, sliced green onion tops, and/or cilantro before serving.