You can use either orange juice or an orange for the sauce. The latter is the preferred way to go but you'll need to supreme the orange for the best results. There's a great demonstration in this Ming Tsai video on the technique.
Which ingredient will reign supreme?
Supremed orange and toasted chopped pecans
Shallot pre-chopped version
Mise en place: altogether now
I know you can just about hear the sizzling
After the fire
Butter, shallots and ...
1 orange supremed along with any juice, or 1/2 cup orange juice
10-12 ounces Pacific sole fillets
1/4 teaspoon salt
1/4 teaspoon freshly ground pepper
2 tablespoons extra-virgin olive oil
1 tablespoon unsalted butter
1 medium shallot, minced
2 tablespoons white-wine vinegar
2 tablespoons pecans, toasted (*) and chopped
2 tablespoons chopped parsley or chopped dill
* Toasting the pecans:
Add the whole pecans to a preheated small iron skillet over medium-high heat. Move the pecans around to avoid burning them. Once they start to get aromatic and colored, remove from the heat to cool.
Directions:If using an orange, supreme it to extract the flesh the pith and membrane into a bowl. Otherwise add the orange juice to a bowl.
Clean and pat dry the sole fillets then season with the salt and pepper. In a large skillet over medium heat add the oil. When hot add the fillets. If you cannot fit them all in the skillet without them touching, cook in batches. Pacific sole is quite delicate and will only take about a minute or so on each side to cook - avoid overcooking, making sure to remove them from the heat as soon as the flesh starts to flake. When the fillets are done, cover with foil while you prepare the sauce.
In the same pan in which you cooked the sole, add the butter to melt. Add the minced shallot and cook until soft and translucent, about 1-2 minutes. Add the vinegar and the orange with its juice using it to loosen any bits stuck in the pan; cook for about 30-60 seconds. Remove from heat and pour over the cooked fillets. Sprinkle the chopped pecans and parsley and serve.