Using a sharp paring knife, remove the skin and white pith from orange. Hold the fruit over a medium bowl and cut between the membranes to release individual orange sections into the bowl, collecting any juice as well. Discard membranes, pith and skin.

Sprinkle both sides of fillets with salt and pepper. Coat a large nonstick skillet with cooking spray and place over medium heat. Add the fillets and cook 1 minute for sole or 3 minutes for tilapia. Gently flip and cook until the fish is opaque in the center and just cooked through, 1 to 2 minutes for sole or 3 to 5 minutes for tilapia. Divide between 2 serving plates; tent with foil to keep warm.

Add butter to the pan and melt over medium heat. Add shallot and cook, stirring, until soft, about 30 seconds. Add vinegar and the orange sections and juice; loosen any browned bits on the bottom of the pan and cook for 30 seconds. Spoon the sauce over the fish and sprinkle each portion with pecans and dill. Serve immediately.