Place the salmon and cod fillets, 2 tablespoons ground red lentils, the olive oil, lemon juice and zest, salt, and pepper into a food processor. Pulse until completely smooth. Add the chopped parsley and coriander leaves and stir well with a spoon. Using your hands, shape the mixture into 10 patties, about ¼-inch thick.
In a bowl, combine the remaining 4 tablespoons ground red lentils with the polenta.
Dip each patty first into the beaten egg, letting any excess drip off, then cover them in the lentil/polenta crumbs.
Heat a large frying pan on medium heat and add about 3-4 tablespoons of olive oil. When the oil is hot, add the patties and cook for about 4 minutes per side, until crisp and golden.
Serve hot with extra parsley sprinkled over top, a side of mayonnaise, and lemon wedges for squeezing.