Season the flank steak with salt and black pepper on both sides.
In a large skillet, heat the olive oil over medium-high heat.
Add the steak and sear for about 4-5 minutes on each side until browned. Remove the steak from the skillet and set aside.
In the same skillet, add the sliced onion and sauté for about 3-4 minutes until softened.
Stir in the minced garlic and cook for an additional minute.
Carefully pour in the Irish whiskey, scraping up any browned bits from the bottom of the skillet. Allow the whiskey to simmer for about 2-3 minutes.
Add the Worcestershire sauce, Dijon mustard, brown sugar, rosemary, and thyme to the skillet. Stir to combine and let the sauce simmer for another 3-4 minutes until slightly thickened.
Return the steak to the skillet, spooning the sauce over it. Cover and cook for an additional 5-7 minutes for medium-rare, or until desired doneness is reached.
Remove the skillet from heat and let the steak rest for 5 minutes.
Slice the steak against the grain and serve with the sauce drizzled on top. Finish with a pat of butter for added richness.
Notes
For a richer flavor, marinate the steak in the whiskey mixture for 1-2 hours before cooking.
If you prefer a sweeter sauce, increase the brown sugar to 2 tablespoons.
Serve with mashed potatoes or crusty bread to soak up the delicious sauce.