Pat beef cubes dry with a paper towel to remove any excess moisture.
Heat oil in a large skillet over high heat. Add steak, working in batches if necessary, and cook 3-4 minutes, stirring constantly, until browned.
Place steak in a 6-quart slow cooker.
Add in black beans, diced tomatoes, onion, red bell pepper, garlic, chili powder, cumin, salt and sugar.
Pour Guinness over mixture in cooker.
Cover slow cooker and cook on low heat for 8 hours.
When there is 10 minutes left of cook time, create a slurry by whisking together water and cornstarch, in a small bowl, until cornstarch is dissolved. Add slurry to slow cooker and stir to combine. Cover and cook on high heat for remainder of cooking time.
Serve chili and top with shredded cheddar cheese, light sour cream, chopped green onion, if desired.
