Method
Season the fillets and cook in a hot pan, skin-side down for approximately three minutes, turn the fillet and finish for one more minute.
Put aside to rest. To cook the cos lettuce, season well and put cut-side down in the pan. Cook for approximately 4 minutes, until the leaves are browned at the edges.
In a separate saucepan, cook the bacon or pancetta pieces until well-coloured and add the peas. Stir the bacon and peas together, over the heat until cooked through.
Place all ingredients except the oil in a food processor. Puree, then add the oil slowly as if making mayonnaise. When smooth, check seasoning and put to one side (can be made a day or two in advance).
Spoon the mayonnaise on a plate and service the fish, cos lettuce, peas and bacon next to it.