This pork is infused with citrus flavor two ways. Orange zest is included in the marinade, and fresh orange juice brightens the final dish. Brining boosts the juiciness of a lean meat, such as pork loin.
Bring water, sugar, and salt to a simmer in a large pot, stirring to dissolve. Remove from heat. Add ice, bay leaf, thyme, orange zest, and peppercorns, and let cool completely. Place pork in a nonreactive dish, and completely cover with brine. Refrigerate at least 6 hours and up to 8 hours. Remove pork from brine, and let stand at room temperature 30 minutes. Pat dry, and season with salt and pepper.
Heat grill to medium. Grill pork, covered with vent open, turning every 6 minutes, until internal temperature reaches 125 degrees, about 32 minutes. (Pork will continue to cook to proper doneness as it rests.) Transfer pork to a cutting board, and let rest 15 minutes. Slice pork, and squeeze orange over slices. Serve with mustard.