Season sirloin with salt, pepper and steak seasoning.
Heat oil in a large frying pan over medium-high. Sear steak on all sides, about 1 minute per side, then place in a slow cooker.
Add 2 cups water and beef bouillon cube to the hot pan. Let the cube dissolve and bring water to a boil.
Whisk remaining 1 cup water with flour, garlic powder, onion powder and black pepper. Slowly whisk into the pan. Continue whisking until gravy thickens and comes to a slow boil.
Remove from heat and carefully pour gravy over steak in the slow cooker. Add diced onion.
Cover and cook on high for 4 hours or until steak is tender.