In a small dish mix 2 tbsp of your olive oil and all seasonings together. Put steak on a plate and brush both sides to coat with this, refrigerate for 1 hour to enhance the flavors.
Take out and preheat oven to 325 F. Heat a cast iron pan over medium high heat and add remaining 1 tbsp olive oil. Sear steak for 2 minutes on both sides, remove, set inside a large piece of foil and wrap up so it is covered.
In the same pan saute the diced onions and minced garlic in the bit of oil left for 3-4 minutes. Then open the foil and pour this on top of the steak. Wrap back up tight with foil.
Place this back into your oven safe pan or on to a baking sheet and bake in the oven for 1.5-2 hours or until the internal temperature reaches 135 F. (for pull apart texture it needs to be much higher at 200 F and would take closer to 4 hours)
Remove from oven and leave sealed for 10 minutes before removing meat and slicing to serve.