Sprinkle beef with garlic powder and sear in vegetable oil over medium heat, about 5 minutes per side. Transfer to a slow cooker.
Dissolve bouillon in hot water, then mix with cornstarch. Pour over the beef in the slow cooker.
Add onion, green peppers, stewed tomatoes, soy sauce, sugar, and salt. Stir to combine.
Cover and cook on High for 3-4 hours or on Low for 6-8 hours.
Serve hot over rice or noodles and enjoy!
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