Season steak with salt and pepper.
Heat a wok or large skillet with tall sides over medium-high heat, and then add oil. Once oil is hot, add steak and sauté for 3 to 4 minutes, stirring occasionally until just browned. Take care to not over-cook.
Stir in green pepper, red pepper, and onion. Sauté, stirring occasionally, until onion is translucent.
Meanwhile, create the sauce. In a small bowl, combine water, sugar, ketchup, rice vinegar, garlic, and red pepper flakes.
Reduce heat to medium and then stir the sauce into the sautéed steak and vegetables. Allow sauce to simmer for a few minutes.
In a small bowl or cup, stir the water and cornstarch until dissolved. Stir the mixture into the steak and vegetables until the sauce thickens.
Serve hot, over rice.
