This is a pretty serious hunk of meat, so to help it cook more evenly, let it sit at room temperature for a while before you put it in the oven.
1 skinless, boneless pork shoulder (Boston butt; about 6 pounds)
1 tablespoon coriander seeds
1 tablespoon crushed red pepper flakes
1 tablespoon dried oregano
1 tablespoon yellow mustard seeds
1/2 cup olive oil
6 garlic cloves, peeled, crushed
1 tablespoon finely grated lemon zest
2 teaspoons smoked paprika
Lightly score fatty side of pork; season all over with salt.
Coarsely grind peppercorns, coriander, red pepper flakes, oregano, and mustard seeds in spice mill or with mortar and pestle; set spice mixture aside.
Heat oil in a small saucepan over low heat; add garlic and cook, stirring, until fragrant and barely golden, about 5 minutes. Stir in lemon zest, paprika, and reserved spice mixture. Let marinade cool.
Rub marinade all over pork, working some marinade into interior of roast. Tie pork at 1″ intervals with kitchen twine. Wrap tightly in plastic and chill at least 8 hours.
Let pork sit at room temperature 1 hour.
Preheat oven to 375°F. Unwrap pork, place on a rack set inside a roasting pan, and roast until golden brown and fat has just started to render, 40–50 minutes. Reduce oven temperature to 300°F and continue to roast until meat is very tender, 1 1/2–2 hours longer.
Transfer pork to a cutting board and let rest 30 minutes before slicing.