Place salmon fillets, skin side down, onto parchment or foil lined baking sheet. Divide chimichurri in half, using ½ cup to evenly coat the salmon filets and reserving remaining ½ cup for serving.
Then top chimichurri-covered salmon with a generous layer of onions and garlic.
Bake until just cooked through, when salmon reaches about 130 degrees or until flesh is set and still pink in the center but can easily be flaked with a fork. About 10-14 minutes depending on thickness of fish.
Remove and carefully lift flesh away from skin. Serve with remaining chimichurri sauce on the side over a bed of mixed greens, with rice and steamed vegetables or use to make chimichurri salmon tacos, burgers or sandwiches.