Bring a large pot of lightly salted water to a full boil. Dice the potatoes into small cubes.
Add the potatoes to the pot and let cook for around 15 minutes, or until easily pierced with a fork (little to no resistance).
In a medium saucepan heated to medium-low, add the butter and cream and allow to cook for 5 minutes. Stir constantly to prevent burning.
Once the potatoes are finished cooking, strain and add to a large bowl along with the cream mixture, salt & pepper, and parsley. Mash everything together until the desired consistency is achieved.
Mix sour cream into the potatoes and top with freshly chopped chives.
Heat a cast iron skillet to high and liberally salt the filet steaks with salt and pepper.
Sear the meat for 2-3 minutes before flipping.
Once flipped, add butter and rosemary to the pan and baste the tenderloin steaks with the hot butter mixture. Let cook for another 2 minutes, then remove from the pan and allow to rest. Do not remove the melted butter from the pan.
Thinly slice the onions and mushrooms and saute in the melted butter until well caramelized.
Add dark beer to the mushrooms and lower heat to medium-low. Allow to simmer for 15-20 minutes, or until the sauce greatly reduces and thickens.
Plate the mashed potatoes, filet mignon, and drunken mushrooms together, top with freshly chopped chives and enjoy!