Heat the olive oil and butter in a large skillet over medium heat.
Add the sliced fennel and sauté for about 5 minutes, until softened.
Add the samphire to the skillet and cook for an additional 2-3 minutes, stirring occasionally.
Season the plaice fillets with salt and black pepper.
Push the vegetables to the side of the skillet and add the fillets.
Cook for 3-4 minutes on each side, or until the fish is cooked through and flakes easily with a fork.
Pour the orange juice and zest over the fish and vegetables in the skillet.
Cook for another minute to warm through.
Serve the plaice topped with the fennel and samphire mixture, garnished with fresh parsley.