Dice 1 small yellow onion.
Heat 1 tablespoon of the vegetable oil in a wok or large nonstick frying pan over medium-high heat until shimmering. Add 2 large eggs and gently scramble with a rubber spatula until just softly set, 1 to 2 minutes. Transfer to a bowl or plate and set aside.
Add the onion, 8 ounces ground pork, 1 teaspoon kosher salt, and ¼ teaspoon black pepper to the pan. Cook, breaking up the meat with a wooden spoon into small pieces, until the pork is cooked through and the onion is tender, about 5 minutes.
Add the remaining 2 tablespoons vegetable oil and let heat for a few seconds. Add 2 cups cold cooked rice, ½ cup frozen peas and carrots, oyster sauce, rice vinegar, and soy sauce or tamari, and stir-fry until the rice is heated through, 2 to 3 minutes. Return the eggs to the pan. Remove from the heat and stir in 1 teaspoon toasted sesame oil.