Mix together the porridge oats, parsley and rosemary with a little salt and pepper, and use this mixture to coat the trout fillets. Heat the oil in a frying pan. Fry the fish in batches for about 3 minutes on each side, or until crispy and golden brown.
Meanwhile, place the shallot, wine and vinegar in a small saucepan and bring to the boil, then leave to bubble until just 1 tablespoon of liquid is left. Remove the pan from the heat and whisk in the butter a little at a time. The residual heat in the pan will melt the butter. Once all the butter has been whisked in, add the lemon juice and season with salt and pepper. Serve immediately with the trout and Lemon Mayonnaise, if liked (see below).