Place the beef in a bowl and sprinkle with the garlic powder, salt, and black pepper. Toss to coat.
Heat the oil in a large skillet over medium high. Add the beef and cook on all sides until the beef is lightly browned, about 5 minutes (no need to cook it all the way through). Transfer the beef and any cooking juices to a 6-quart slow cooker.
To the skillet where you were just cooking the beef, add the onions. Increase the heat to high. Carefully pour in 1/4 cup water, stirring to scrape up any browned bits stuck on the pan. Sauté until the onions begin to soften and lightly brown and the liquid has cooked off, about 3 minutes, then transfer the onions to the slow cooker with the beef.
To the slow cooker, add the green and red bell peppers (see recipe note) and tomatoes. In a small bowl or a large liquid measuring cup, whisk together the soy sauce, Worcestershire sauce, honey, ginger, red pepper flakes, and 3 tablespoons cornstarch. Pour into the slow cooker.
Cover and cook on LOW for 6 to 7 hours, until the beef is tender (you can also make this recipe on HIGH, cooking it for 3 to 4 hours, though I find the beef comes out more tender on low heat).
In a small bowl, whisk together the remaining 2 tablespoons cornstarch with the remaining 2 tablespoons water to make a slurry. Stir the slurry into the slow cooker. Turn the slow cooker to HIGH and let cook, uncovered, for 10 minutes to further thicken. The pepper beef will still be saucy, but it should resemble a thick, rich gravy. Stir once more. Taste and add additional salt, soy sauce, and/or red pepper flakes to taste. Serve hot with prepared brown rice and a sprinkle of green onion.
The bell peppers will become quite soft during cooking; if you prefer them more crisp, wait to add them a little more than halfway through the cook time.
TO STORE: Place leftover steak in an airtight storage container in the refrigerator for up to 3 days.
TO REHEAT: Gently rewarm leftovers in a large skillet on the stove over medium-low heat. You can also reheat this dish in the microwave.
TO FREEZE: Store pepper steak in an airtight freezer-safe storage container in the freezer for up to 3 months. Let thaw overnight in the refrigerator before reheating.