Chop parsley and cilantro. Mince shallot and garlic until fine.
Stir herbs with oregano, red pepper flakes, vinegar, lemon juice, salt, and pepper.
Drizzle in olive oil while stirring until emulsified. Add warm water to loosen if desired. Let sit 10 minutes.
Pat steak dry. Season all over with salt and pepper, then rub lightly with olive oil.
Heat a cast-iron skillet or grill over high until very hot.
Sear steak 3–4 minutes per side for medium-rare. Add butter in the last minute and baste, if using.
Rest steak 5–10 minutes. Slice against the grain and spoon chimichurri over the top.