1. Preheat oven to 250°. Heat a cast-iron ridged grill pan over high heat until very hot. Trim any fat from roast, then sprinkle generously with kosher salt. Wipe pan with an oiled paper towel, then sear beef on each side until a nice brown crust forms, 10 to 12 minutes total.
2. Put pan of beef in oven and roast until meat registers 130° in center when tested with an instant-read thermometer, 1 to 1 1/2 hours. Let rest 20 minutes. Slice paper-thin across the grain and serve with salsa verde.