*Pork butt is often labeled Boston butt in the supermarket. Look for a roast with a substantial fat cap. If fennel seeds are unavailable, substitute 1/4 cut ground fennel. The Porchetta needs to be refrigerated for 6 to 24 hours once it is rubbed with the paste, but it is best when it sits for a full 24 hours.*

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 tbsp pepper, and 2 tsp salt. Pulse mixture until finely chopped, 10 to 15 pulses. Add oil and process until smooth paste forms, 20 to 30 seconds

Using a sharp knife, cut slits in surface fat of roast, spaced 1 in apart, in crosshatch pattern, being careful not to cut into meat. Cut roast in half with the 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 in from short end of each roast, use boning or paring knife to make slit that starts 1 in from top of roast and ends 1 in from bottom, pushing knife completely through roast. Repeat making slits, spaced 1 to 1 1/2 in apart, along length of each roast, stopping 1 in 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 tsp 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 tbsp salt, 1 tsp 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 hr or up to 24 hr.

Adjust oven rack to middle position and heat oven to 325 degrees. Transfer roasts, fat side up, to large, high-rimmed roasting pan, leaving 2 inches between roasts. Cover pan tightly with aluminum foil. Cook until pork registers 180 degrees, about 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 a 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.