Heat a large, well-seasoned cast iron skillet over high heat until very hot, 5 to 7 minutes.
Meanwhile, pat the steaks dry with a paper towel and brush them lightly with vegetable oil. Combine the salt and cracked pepper in a shallow bowl and roll the steaks in the mixture, pressing lightly to evenly coat all sides.
When the skillet is ready, add the steaks and sear them evenly on all sides for about.
1 - to -1 1/2 minutes per side.
Top each steak with a tablespoon of butter and place the skillet in the oven. Cook the steaks until they reach 120 degrees F for rare or 125 degrees F for medium-rare on an instant-read thermometer. (To test the steaks, insert the thermometer sideways to be sure you're actually testing the middle of the steak.).
Remove the steaks to a serving platter, cover tightly with aluminum foil and allow to rest at room temperature for 10 minutes.
Brush the caps of each mushroom with a clean sponge or damp paper towel. Remove and discard the stems. Slice the small mushrooms thickly and cut the large ones in a large dice.
Heat the olive oil in a large (11 - inch) Dutch oven or saucepan. Add the leeks and cook over low heat for 5 minutes, or until the leeks are translucent. Add the butter, mushrooms, salt, and pepper and cook over medium heat for 8 minutes, until they are tender and begin to release their juices, stirring often. Stir in the garlic and cook for 2 more minutes. Toss in the parsley, sprinkle with salt, and serve warm.