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1
Marinate the steak.
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2
Poke the steak all over with the tines of a fork; cut in half, against the grain, to make 2 steaks (one steak may be slightly thicker).
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3
In a dish large enough to hold the steaks, combine the olive oil, wine vinegar, shallot, garlic, and the thyme.
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4
Season with pepper to taste.
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5
Add the steaks and turn to coat evenly.
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6
Cover, and marinate the steaks at room temperature for 1 hour, turning once, or refrigerate, for up to 12 hours.
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7
Make the mushroom sauce.
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8
Heat a medium saute pan over high heat, add the butter, and when the foam subsides, add the mushrooms.
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9
Reduce the heat to medium-high and cook, the mushrooms, tossing frequently, until browned, about 5 minutes.
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10
Season with the 1/2 teaspoon salt and pepper to taste.
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11
Add the garlic, shallot, thyme, and rosemary, and cook for 2 minutes more.
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12
Pour the red wine into pan, scraping up any browned bits on the bottom of the pan with a wooden spoon.
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13
Cook until almost all the liquid has evaporated.
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14
Add the cream, parsley, and chives and stir to combine.
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15
Set aside.
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16
Heat a large grill pan over medium-high heat or an outdoor grill to medium-high.
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17
Remove the steaks from the marinade and pat dry.
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18
Season with salt and pepper.
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19
Cook the steaks for 3 to 5 minutes on each side for rare to medium-rare.
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20
Set aside on a cutting board for 5 minutes.
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21
Cut the steak against the grain on an angle into very thin slices.
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22
Arrange the slices on a warmed platter or plates.
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23
Reheat the mushroom sauce if needed and add any accumulated meat juices.
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24
Pour the sauce over the steak and serve immediately.
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25
Cook's Note: Flank steak is the original cut for a London Broil.
