Let beef sit at room temperature for 10 minutes, then freeze for 15 minutes to firm up. Slice against the grain into thin strips about 1/4-inch thick, trim excess fat, and season lightly with salt and pepper.
Remove seeds and ribs from bell peppers and slice into 1/2-inch strips. Peel and halve the onion, then slice into thin half-moons. Mince the garlic finely.
In a bowl, whisk together beef broth, soy sauce, brown sugar, Worcestershire sauce, grated ginger, and minced garlic until the sugar dissolves completely. Taste and adjust seasoning as needed.
Lightly grease the slow cooker insert, then layer in this order: half the onions on the bottom, beef strips, remaining onions, bell pepper strips, drained diced tomatoes, and finally the sauce poured evenly over everything.
Cover and cook on LOW for 6–8 hours or HIGH for 3–4 hours without lifting the lid. The beef is done when it's fork-tender and easily pulls apart.
About 30 minutes before serving, mix cornstarch with 2 tablespoons cold water to make a slurry and stir it into the crockpot. Cook on HIGH for 20–30 minutes until the sauce reaches a gravy-like consistency.
Cook rice according to package directions, fluff with a fork, and divide among bowls. Top with pepper steak and extra sauce, then garnish with sliced green onions. Serve immediately.