Reverse Sear Rib-Eye Roast With Fennel and Rosemary

Working in 2 batches, scatter rosemary over a paper towel–lined plate. Top with another sheet of paper towel and microwave on high 1 minute. Check rosemary and continue to microwave in 20-second intervals, until dry and crisp. Grind rosemary in spice mill or with mortar and pestle to a powder. Transfer to a small bowl and mix in fennel pollen.
Place roast, fat side down, on a cutting board with a long side facing you. Using a sharp slicing knife and long strokes, cut roast into two 3-bone roasts. French rib bones 1 1/2" from tip of bones, if desired. Score fat cap in a crosshatch pattern, spacing cuts 1" apart. Rub roasts all over with garlic, then coat with rosemary mixture. Season generously with salt and pepper. Place, fat cap up, on a rack, set inside a roasting pan. Chill, uncovered, at least 8 hours and up to 1 day.
Let roasts sit at room temperature 2 hours for even roasting.
Place a rack in lower third of oven; preheat to 225°F. Roast meat, rotating pan once, until an instant-read thermometer inserted into the thickest part of roasts registers 125°F for medium-rare, or 130°F for medium, 3 ½–4 hours. Remove from oven, tent with foil, and let rest 30 minutes.
Meanwhile, cook bacon in a small saucepan over medium heat, stirring occasionally, until brown and crisp, 8–10 minutes. Using a slotted spoon, transfer bacon to a small bowl; reserve for another use (it’s the fat you’re after). Add butter pieces to drippings in pan and cook, stirring often, until mixture foams, then browns, 5–8 minutes. Strain into a medium nonreactive bowl. Immediately stir in shallot; let cool (butter will have set up but will still be soft). Using an electric mixer on medium-high, add room-temperature butter to bacon fat mixture and beat until light and fluffy. Mix in chives and vinegar; season bacon butter with salt. Chill, stirring occasionally, at least 20 minutes (butter should be a little more firm but still light and fluffy).
Move rack to upper third of oven; increase oven temperature to 500°F. Roast meat until deep brown and exterior is crisp, 8–10 minutes. Serve rib-eye roast with bacon butter.
Bacon butter mixture (without chives and vinegar) can be made 3 days ahead. Cover and keep chilled. Let soften slightly before adding chives and vinegar.