Sprinkle the sirloin strips with garlic powder to taste. In a large skillet, heat the vegetable oil over medium heat, and brown the seasoned beef strips. You may need to do this in two batches. Transfer to a slow cooker.
In a small bowl or cup, dissolve the bouillon cube in hot water. Stir in cornstarch, until well combined. Pour mixture into the slow cooker with meat. Add chopped onions, stewed tomatoes with liquid, soy sauce, sugar, and salt; stir to combine.
Cover, and cook on HIGH for 3 to 4 hours, OR on LOW for 6 to 8 hours.
The last 30 minutes of cooking, add the sliced peppers and turn heat up to high (if it wasn't already on high).