How to Make It
Roast the chiles over a gas flame or broil them, turning with tongs, until charred all over, about 10 minutes. Transfer to a bowl and cover with plastic wrap; let steam for 15 minutes. Slip off and discard the skins. Stem and seed the chiles, then thinly slice lengthwise.
Meanwhile, in a large cast-iron skillet, melt 1 tablespoon of the butter in 2 tablespoons of the oil. Add the onions, season with salt and pepper and cook over moderate heat, stirring occasionally, until they are deep golden, about 15 minutes. Stir in the roasted chiles and the remaining 1 tablespoon of butter. Keep warm.
Heat a grill pan. Rub the steaks with the remaining 2 tablespoons of oil; season with salt and pepper. Cook over moderately high heat, turning once, until nicely charred and medium-rare, about 3 minutes per side. Transfer to plates and let rest for 5 minutes. Serve with the chile-onion mixture.