Prepare the Chimichurri:
In a small bowl, combine the shallot, garlic, parsley, cilantro, red pepper flakes, salt, and black pepper.
Stir in the olive oil and red wine vinegar. Let it sit at room temperature while you cook the steak to allow flavors to meld.
Cook the Steak:
Remove the skirt steak from the fridge and let it come to room temperature for about 20 minutes.
Season both sides with salt and black pepper.
Sear the Steak:
Heat a HexClad or cast iron skillet over medium-high heat until hot.
Add the steak and sear for 2-3 minutes per side, until a golden crust forms.
Baste for Flavor:
Lower the heat to medium and add butter, smashed garlic, and rosemary to the pan.
Tilt the pan slightly and spoon the melted butter over the steak repeatedly for about 1 minute.
Rest and Slice:
Transfer the steak to a cutting board and let it rest for 5 minutes before slicing against the grain.
Serve:
Arrange the sliced steak on a serving platter and generously drizzle with chimichurri.
Enjoy immediately with your favorite sides!