Add the oil to a large saute pan over high heat and wait for it to lightly smoke. Add in the potatoes and onions and cook for 8-10 minutes or until you begin to see some dark brown edges on the potatoes and onions.
Season the potatoes with salt and pepper and add in 1/2 the butter over top. Transfer the pan to the oven and cook for 10-12 minutes or until they are browned and cooked through. Finish by deglazing with the white wine vinegar and remaining butter. Mix before serving.
Steak: Season the steaks generously on both sides with salt and pepper.
Add the oil to a large cast iron skillet or fry pan and wait for it to lightly smoke. Add in the steaks and turn the heat down to medium-high. Add in the herbs, garlic and butter.
Cook the steak for 5-6 minutes and then flip it over and immediately begin basting it for 2 to 3 minutes.
Remove the steaks and let them rest before slicing and serving alongside the potatoes.