Remove excess silver skin and large pockets of fat from the chuck flap tail, leaving a thin layer for flavor. Pat the meat dry with paper towels and score the surface lightly (shallow diagonal cuts) to help the marinade penetrate.

Make the marinade

In a bowl, whisk together minced garlic, soy sauce, Worcestershire sauce, red wine vinegar, olive oil, chopped rosemary, thyme, brown sugar, crushed red pepper flakes (if using), 1 tablespoon kosher salt and 1 teaspoon freshly ground black pepper until combined.

Marinate

Place the trimmed chuck flap tail in a resealable plastic bag or nonreactive container. Pour the marinade over the beef, pressing out excess air and ensuring the meat is evenly coated. Refrigerate for 2–6 hours (up to overnight for deeper flavor). If marinating longer than 4 hours, flip the meat once halfway through.

Remove from fridge and season

About 30–45 minutes before cooking, remove the meat from the refrigerator to come close to room temperature. Discard the excess marinade. Pat the meat dry again and season both sides with the remaining 1 tablespoon kosher salt and 1 teaspoon black pepper.

Preheat grill or pan

If grilling: preheat to high (450–500°F / 230–260°C) and oil the grates. If using a cast-iron skillet: place the skillet over high heat until smoking hot and add a thin film of high-smoke-point oil (e.g., canola).

Sear and cook

Place the chuck flap tail on the hot grill or skillet. Sear without moving for 3–4 minutes until a deep brown crust forms. Flip and sear the other side for 3–4 minutes. For medium-rare aim for an internal temperature of 55–57°C (130–135°F). For medium 60–63°C (140–145°F). Use an instant-read thermometer inserted into the thickest part.

Carryover cooking

If the steak reaches target temperature early or has uneven thickness, move it to a cooler part of the grill or reduce heat and continue to cook gently until temperature is reached. Remove the steak from heat 3–4°C (5–7°F) below desired final temperature to account for carryover.

Finishing & Serving

Rest and finish

Transfer the cooked chuck flap tail to a cutting board and tent loosely with foil. Let it rest for 8–10 minutes to allow juices to redistribute.

Butter baste and slice

While resting, melt unsalted butter and stir in fresh lemon juice and half the chopped parsley. Pour the butter mixture over the steak. Slice the steak thinly across the grain into 1/4-inch (6 mm) slices.

Serve

Arrange slices on a warm platter, sprinkle with remaining chopped parsley, and serve with optional chimichurri or steak sauce on the side. Suggest serving with roasted potatoes, grilled vegetables, or a simple green salad.