In a gallon-size zip-top bag, place roast, onion, peppers, 1 cup water and garlic. Label, and freeze.
When ready to cook, thaw in refrigerator 1-2 days.
Place contents of bag into slow cooker. Cover, and cook on low 6-8 hours.
Remove roast from slow cooker. Transfer vegetables and juices to a large saucepan, and bring to a boil. In a small bowl, mix cornstarch, soy sauce and sugar until smooth; stir into vegetable mixture. Return to a boil, stirring constantly; cook until thickened, about 1-2 minutes.
Cut beef into slices or shred, and return to slow cooker. Pour vegetable mixture over beef, and stir gently. Replace the lid on the slow cooker, and let heat 30 minutes. Serve with rice or mashed potatoes, if desired.
Notes
This Crockpot Pepper Steak recipe is written as a freezer meal. If making immediately, place roast, onions, peppers, garlic and water in slow cooker and follow remaining serving day cooking instructions.