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1
Bring onion, garlic, bay leaf, salt, brown sugar, coriander seeds, fennel seeds, peppercorns, and 2 cups water to a boil in a medium saucepan. Reduce heat and simmer, stirring, until salt and sugar are dissolved. Remove from heat and add 2 cups cold water. Transfer brine to a large heatproof bowl or jar. Squeeze orange and lemon juice into brine and add citrus halves and thyme. Chill until cool.
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2
Place pork in a large resealable plastic bag and add brine. Seal bag, pressing out air, and chill at least 12 hours.
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3
Do ahead: Brine can be made 2 days ahead; cover and keep chilled. Pork can be brined 24 hours ahead; keep chilled.
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4
Remove pork from brine, brushing off any spices. Let stand at room temperature 1 hour before grilling.
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5
Toast coriander and fennel seeds in a small skillet over medium heat, tossing occasionally, until fragrant, about 2 minutes. Let cool, then finely grind in mortar and pestle or spice mill; combine in a small bowl with brown sugar, salt, pepper, and paprika.
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6
Prepare grill for 2-zone heat (for a charcoal grill, bank coals on 1 side of grill; for a gas grill, leave 1 or 2 burners off) and oil grate. Pat pork dry and rub all over with dry rub. Grill over direct heat, bone side up, until lightly charred, about 5 minutes. Turn pork and grill, bone side down, until lightly charred, about 5 minutes more.
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7
Transfer pork to cooler side of grill, bone side up; cover grill. Grill pork, turning occasionally, until an instant-read thermometer inserted into the center of loin registers 145u00b0, 45-60 minutes. (The internal temperature of grill should be 400u00b0; add more charcoal or adjust heat as needed.)
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8
Transfer pork to a cutting board, tent loosely with foil, and let rest 20 minutes. Cut loin from ribs, and slice loin; cut between ribs. Serve with Peach Mustard.
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9
Do ahead: Dry rub can be made 1 day ahead. Store airtight at room temperature.
