Flatten the steaks with a meat mallet so they are about 1 1/2 inches thick. Season the steaks with salt and pepper on both sides.
Heat the olive oil and butter in a large sauté pan over medium-high heat. When the oil is hot and the butter has melted, add the steaks and sear until well browned, 3 to 4 minutes per side. Set the steaks aside on a plate.
To the same pan, add the garlic and shallot and cook, scraping the bottom of the pan to loosen any of the flavorsome steak residue into the mixture, for about 1 minute. Remove the pan from the heat and carefully add the brandy. (If the pan ignites, stand back and allow the flame to disappear, which it will in a few seconds.) Return the pan to the heat and bring to a simmer, scraping the bottom of the pan again. Simmer until the brandy is almost totally reduced, about 2 minutes. Add the broth, cream, mustard and Worcestershire sauce and stir to combine, then cook for a minute to thicken.
Plate up the steak and pour over the sauce. Garnish with the parsley and chives and serve immediately.