1. Pat steaks dry, season with salt and pepper on both sides.
2. Preheat oven to 425F (220C).
3. Heat olive oil in a large skillet over medium-high heat. Add steaks and sear on one side until nicely browned, about 3 minutes.
4. Transfer steak to a foil-lined baking sheet, seared side up. Spread about 1/2 teaspoon mustard on each steak (reserve remaining for sauce) and top with parsley.
5. Transfer to the oven and bake for about 8 minutes (until the internal temperature is at least 145F (63C)).
6. Return the skillet to medium heat. Add heavy whipping cream and remaining mustard, cook, stirring, until thickened, 2-3 minutes. Remove from heat, season with salt and pepper, stir in butter.
7. Pour some sauce on a plate, place steak on top and serve.