Prepare the Steak: Start by seasoning the hanger steak generously with salt and pepper on both sides. Let it sit at room temperature for about 30 minutes.

Sear the Steak: In a large, heavy-bottomed skillet or Dutch oven, heat the olive oil over medium-high heat. Once hot, add the hanger steak and sear for about 4-5 minutes on each side, or until browned. Remove the steak from the skillet and set it aside on a plate.

Caramelize the Onions: In the same skillet, add the sliced onions. Reduce the heat to medium and cook, stirring occasionally, for about 15-20 minutes, or until the onions are deeply caramelized and golden brown. Add the minced garlic and cook for another 2-3 minutes until fragrant.

Deglaze the Pan: Pour in the red wine, scraping the bottom of the skillet to release any browned bits. Bring the mixture to a simmer and let it cook for about 5 minutes to reduce slightly.

Add Broth and Seasonings: Stir in the beef broth, Worcestershire sauce, tarragon, and thyme. Return the seared hanger steak to the skillet, nestling it into the onion mixture.

Braise the Steak: Cover the skillet with a lid and reduce the heat to low. Let the steak braise for about 1.5 to 2 hours, or until it is fork-tender. Check occasionally to ensure the liquid does not evaporate too much; add more broth if needed.

Finish the Gravy: Once the steak is tender, carefully remove it from the skillet and let it rest on a cutting board. Increase the heat to medium-high and let the gravy simmer uncovered for about 5-10 minutes until slightly thickened. Stir in the butter for a rich finish.

Serve: Slice the hanger steak against the grain. Serve it on a platter topped with the caramelized onion and herb gravy. Garnish with freshly chopped parsley.