Season the sirloin steak with salt and pepper.
Heat oil in a large frying pan over medium high heat and sear steak on all sides.
Place sirloin steak in a slow cooker.
Add 2 cups of water and the beef bouillon cube to the hot pan.
Allow bouillon cube to dissolve and water to come to a boil.
In a small mixing bowl whisk together 1 cup of water, flour, garlic powder, onion powder and pepper.
Slowly pour flour mixture into the pan. Whisk in the flour mixture.
Continue to whisk and let the gravy thicken and come to a slow boil.
Remove from heat and carefully pour the gravy over the steak in the slow cooker.
Add diced onions to the slow cooker.
Cover and cook on High for 4 hours or until steak is tender.