Use your fingers to pull the leaves of rosemary off of the stem. In a medium-sized bowl, add red wine, rosemary leaves, and salt. Add the filets. Cover the bowl with aluminum foil and put it in the fridge for about an hour, or overnight.
Add 1 tbsp butter into a saucepan over medium heat. Once the butter has melted, add the chopped onion and saute until completely cooked. Add the whole allspice and stir.
Add the white wine and heavy whipping cream into the saucepan and stir again. Bring to a simmer over medium heat and allow to cook for about 10 minutes until it has reduced in half. Set aside.
Preheat the oven to 400 degrees. Take the steaks out of the fridge.
In a cast iron skillet, put 1 tbsp of butter. Put the cast iron skillet on very high heat and heat until the butter is almost smoking (but do not burn it).
Once the cast iron is heated up enough, put the steaks in the pan and sear for about 2 minutes until a crust forms on the outside of the steak. Use tongs to turn them over and sear for 2 minutes on the other side. This will get kind of smokey.
Once you have seared both sides, put the cast iron skillet directly in the preheated oven (using oven mitts if necessary). Cook for 4 minutes for rare, 5-6 minutes for medium rare, 6-7 minutes for medium, and 8-9 minutes for medium well.
Remove the steaks from the oven and transfer to a plate. Lightly cover them with tin foil and allow them to sit for 5 minutes before serving.
Transfer to individual plates and pour cream sauce over the top. Enjoy!