In a bowl, whisk together the orange zest, orange juice, lemon juice, olive oil, fennel seeds, garlic, chopped rosemary, red pepper flakes, mustard, salt and black pepper. Pour the marinade into a large resealable plastic bag. Add the pork tenderloins, seal the bag and refrigerate for 2 hours.
Remove the pork from the marinade and discard the marinade. Preheat the grill or use a cast-iron grill pan preheated over medium-high heat until hot, about 5 minutes. Place the pork on the grill pan and cook, turning occasionally, until nicely grill-marked and an instant-read thermometer inserted into the center of the meat registers 140°F, 25 to 30 minutes. Transfer the pork to a cutting board, cover loosely with aluminum foil and let rest for 10 minutes.
Meanwhile, place the orange slices on the grill pan and cook, turning once, until nicely grill-marked, 1 to 2 minutes per side. Cut the pork into slices and arrange on a warmed platter. Garnish with the orange slices and rosemary sprigs and serve immediately.