Combine maple syrup, balsamic vinegar, and dijon mustard with orange juice and set aside.
Heat an oven-safe frying pan over medium-high heat. Add coconut oil. Then add the salmon. It will sear quickly. Flip it over, and turn off the heat. Leaving the fish in the pan, pour the orange juice mixture over it. Sprinkle on chopped onion.
Move the hot pan to the oven and cook, uncovered, for 6 to 9 minutes for the salmon to finish cooking inside. The juice mixture should be reduced to a nice glaze. Watch it closely to make sure it doesn't boil dry and burn.
Serve with the glaze scraped from the pan and drizzled over the salmon. Leftovers keep well in the fridge and are nice served on a lettuce salad.