Make sure the beef is at room temperature before cooking. Transfer the bone-in beef entrecôte directly to an oven rack positioned over a baking sheet in a preheated oven and bake at 50°C/120°F for approx. 2 hrs., flipping once.
When internal temperature reaches 50°C/120°F, remove steak from the oven and transfer to a preheated grill pan over high heat. Sear for approx. 2 min. per side. Add rosemary and butter, then carefully tilt pan and baste steak in melted butter with a spoon. Remove steak from pan and transfer to an aluminum foil-lined plate with rosemary and wrap in aluminum foil and let rest for approx. 5 min.
Remove from aluminum foil and transfer to a cutting board. Slice off the bone and into thick pieces. Serve with rosemary and plenty of fleur de sel. Enjoy!