To make salsa: With a sharp knife, remove skin and white pith from oranges and discard. Working over a medium bowl to catch the juice, cut the orange segments from their surrounding membrane, letting the segments fall into the bowl. Add onion, lime juice, cilantro, chipotle peppers and garlic. Stir to combine. Season with salt and pepper.
To make salmon: Heat a small skillet over medium heat. Add coriander and cumin; toast, stirring constantly, until aromatic, 2 to 3 minutes. Let cool and transfer to a spice mill or mortar and pestle. Add peppercorns to the mill and coarsely grind all the spices. Stir in salt.
Preheat grill or broiler.
Coat salmon with the spice mixture. Grill or broil on a lightly oiled rack until the fish is opaque in the center, about 5 minutes per side. Serve with the salsa.
Make Ahead Tip: Cover and refrigerate the salsa (Step 1) for up to 8 hours.