Lightly grease a 2-quart baking dish or spray with cooking spray. Heat oven to 325°.
In a large skillet over medium-high heat, heat the olive oil. Sprinkle the steaks with grill seasoning and brown lightly on both sides. Remove the steaks to a plate.
Add onions to the same skillet and cook, stirring frequently, until lightly browned, about 4 to 6 minutes.
Combine ketchup with 1 tablespoon steak seasoning, vinegar, brown sugar, mustard, and Worcestershire sauce.
Pour about 1/2 cup of the sauce in the baking dish. Arrange about 1/4 of the onions over the sauce. Arrange some of the steaks over the sauce, spoon more of the sauce over each steak and sprinkle with half of the remaining onions. Repeat with the remaining steaks, topping with the remaining onions and sauce.
Cover the baking dish tightly with foil and bake for 1 1/2 hours.