Slice 3 pounds of flank steak into thin strips across the grain. Place the steak in a zip-top bag with 1/2 cup cornstarch, seal, and shake until evenly coated.
In the slow cooker, combine 3 tablespoons olive oil, 1 cup brown sugar, 1 1/2 cups soy sauce, 1 1/2 cups water, and 2 minced garlic cloves. Stir until sugar dissolves. Add the coated beef strips and spread evenly in the sauce.
Cover and set the slow cooker on low for approximately 6 hours. Avoid lifting the lid frequently to retain heat.
About 1 1/2 hours before the end, slice 4 bell peppers and 2 onions into thin strips. Add them to the slow cooker, cover again, and cook for the remaining time.
Once done, serve the pepper steak over steamed rice, noodles, or mashed potatoes. Enjoy the flavorful sauce and tender beef!
Notes
For extra flavor, add a dash of sesame oil and garnish with green onions just before serving.