I think the simpler you cook fish the better, pan seared or roasted in the oven with extra virgin olive oil and butter to finish, with a good squeeze of lemon juice. Lemon sole and plaice , nothing nicer when fresh and good thick fillets.
The key to cooking fillets of flat fish is a medium/hot pan, extra virgin olive oil to start. Dust fillets lightly with seasoned flour and cook skin side down, turning only when skin is golden and crisp and upper side slightly opaque. Then turn over gently, add a knob of butter and move pan off heat to continue cooking in residual heat. A squeeze of lemon juice to finish and freshly chopped parsley or chives, delicious!