Using sous vide circulator, bring water to 145°F/63°C in 12-quart container.
Grind fennel seeds in spice grinder or mortar and pestle until finely ground. Transfer ground fennel to food processor and add rosemary, thyme, garlic, 1 tablespoon pepper, and 2 teaspoons salt. Pulse mixture until finely chopped, 10 to 15 pulses. Add oil and process until smooth paste forms, 20 to 30 seconds.
Using sharp knife, cut slits in surface layer of fat on roast, spaced 1 inch apart, in crosshatch pattern, being careful not to cut into meat. Cut roast in half with grain into 2 equal pieces.
Turn each roast on its side so fat cap is facing away from you, bottom of roast is facing toward you, and newly cut side is facing up. Starting 1 inch from short end of each roast, use boning or paring knife to make slit that starts 1 inch from top of roast and ends 1 inch from bottom, pushing knife completely through roast. Repeat making slits, spaced 1 to 1½ inches apart, along length of each roast, stopping 1 inch from opposite end (you should have 6 to 8 slits).
Turn roast so fat cap is facing down. Rub sides and bottom of each roast with 2 teaspoons salt, taking care to work salt into slits from both sides. Rub herb paste onto sides and bottom of each roast, taking care to work paste into slits from both sides. Flip roast so that fat cap is facing up. Using 3 pieces of kitchen twine per roast, tie each roast into compact cylinder.
Combine 1 tablespoon salt and 1 teaspoon pepper in small bowl. Rub fat cap of each roast with salt mixture, taking care to work mixture into crosshatches. Place each roast in 1-gallon zipper-lock freezer bag. Seal bags, pressing out as much air as possible. Gently lower bags into prepared water bath until roasts are fully submerged, and then clip top corner of each bag to side of water bath container, allowing remaining air bubbles to rise to top of bag. Reopen 1 corner of zipper, release remaining air bubbles, and reseal bag. Cover and cook for at least 20 hours or up to 24 hours.
Adjust oven rack to middle position and heat oven to 500°F/260°C. Set wire rack in aluminum foil–lined rimmed baking sheet and spray with vegetable spray. Transfer roasts, fat side up, to prepared rack, leaving at least 2 inches between roasts. Discard twine and pat roasts dry with paper towels. Roast until exteriors of roasts are well browned, about 20 minutes.
Transfer roasts to carving board and let rest for 5 to 10 minutes. Slice roasts ½ inch thick and serve.