In a large cast iron skillet, heat the oil over med/high heat. Season the salmon with salt and pepper. Place the salmon in the skillet, skin side down (if your salmon doesn't have skin, just place in the pan). Sear the salmon for 2-3 minutes or until the bottom loosens from the pan easily and is golden brown. Transfer the skillet to the oven for 10-15 minutes or until the fish flakes easily with a fork.
While the salmon is roasting, toss the spinach, onion, oranges, orange juice and olive oil until combined. Season with salt and pepper.