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 fat of 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 1 1/2 inches apart, along length of each roast, stopping 1 inch from opposite end (you should have 6 to 8 slits, depending on size of roast).
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, 1 teaspoon pepper, and baking soda in small bowl. Rub fat cap of each roast with salt–baking soda mixture, taking care to work mixture into crosshatches. Transfer roasts to wire rack set in rimmed baking sheet and refrigerate, uncovered, for at least 6 hours or up to 24 hours.
Adjust oven rack to middle position and heat oven to 325 degrees. Transfer roasts, fat side up, to large roasting pan, leaving at least 2 inches between roasts. Cover tightly with aluminum foil. Cook until pork registers 180 degrees, 2 to 2 1/2 hours.
Remove pan from oven and increase oven temperature to 500 degrees. Carefully remove and discard foil and transfer roasts to large plate. Discard liquid in pan. Line pan with foil. Remove twine from roasts; return roasts to pan, directly on foil; and return pan to oven. Cook until exteriors of roasts are well browned and interiors register 190 degrees, 20 to 30 minutes.
Transfer roasts to carving board and let rest for 20 minutes. Slice roasts 1/2 inch thick, transfer to serving platter, and serve.