Prepare the beef stir fry marinade by whisking soy sauce, sesame oil, and brown sugar together. Set aside.
Add a bit of oil to a large preheated skillet. Add in your veggies. Saute them over high-heat until they soften and turn light brown. Remove veggies from the skillet and set aside.
In the same skillet, brown the ground beef until it's fully cooked. Return the veggies to the pan. Cover the stir fry with the marinade. Stir to combine.
Cook the stir fry for about 2-3 more minutes and remove from it from heat. Serve over steamed white rice.