Place a steaming basket over a pot of water and turn on high heat.
Generously season steaks with salt and pepper. Add oil to a cast iron skillet and heat on high until smoking. Add the steaks to the pan (you may need to do this in batches). Cook until both sides of the steaks are well browned about 3 minutes per side. Make sure to roll the edges of the steaks so they get a nice caramelized crust also. Cook the steak until probe thermometer reaches 48C, about 5-10 minutes.
Remove the steaks from the plan, flip the steaks, and let rest on rack in a warm place for about 10 minutes.
In the steak pan on high heat, add 2 tbsp of butter and then the mushrooms, cook until mushrooms are nice and caramelized. Move mushrooms to one side of the pan and add the shallots. Allow the shallots to sweat to remove their raw flavour. Now toss the mushrooms with the shallots, deglaze with the cognac and then reduce liquid until almost gone. Add the pre-made demi-glace, adjust seasoning with salt and pepper, and let cook on low.
Place lobster in steamer and let cook for about 7 minutes until warmed through.