Cut the flank steak into thin strips and place them in a large ziplock bag.
Add the corn starch to the bag, seal it, and shake well to coat the steak evenly.
Heat the olive oil in a large skillet over medium-high heat. Add the coated steak to the skillet and brown it on all sides, stirring occasionally. This should take about 5 minutes.
In a separate bowl, mix together the minced garlic, soy sauce, water, and brown sugar until well combined.
Transfer the browned steak to a slow cooker and pour the sauce over it. Cover the slow cooker and cook on low heat for 6 hours.
After 4 1/2 hours, add the sliced onions, green peppers, and red peppers to the slow cooker. Mix well to combine the ingredients.
Cover the slow cooker and continue cooking for an additional 1 1/2 hours, until the vegetables are tender and the steak is fully cooked.
