Heat 1 tablespoon vegetable oil in a large skillet or wok over medium heat.
Add the beaten eggs and scramble until just set; transfer the scrambled eggs to a plate and set aside.
Increase heat to medium-high and add the remaining 1 tablespoon vegetable oil.
Season the sliced beef with salt and pepper, add to the hot pan, and stir-fry until browned and cooked through, about 3–4 minutes; remove and set aside.
In the same pan, add the diced onion and minced garlic and stir-fry until the onion is translucent, about 2 minutes.
Add the thawed peas and carrots and cook for about 3 minutes until heated through.
Add the cooked, cooled brown rice to the pan and stir to combine with the vegetables.
Whisk together the low-sodium soy sauce, oyster sauce, and sesame oil in a small bowl; pour the sauce over the rice and stir-fry for 3–4 minutes to heat through and coat the rice.
Return the cooked beef and scrambled eggs to the pan, stir to combine, and cook for an additional 2 minutes until everything is heated through.
Remove from heat, garnish with sliced green onions, and serve hot.
