In a shallow dish, stir together 1/4 cup of lime juice and beer. Place fish in the dish and turn to coat. Marinate for 30 minutes.
Preheat the oven to 200 degrees C. Coat a roasting rack with cooking spray and place over a baking tray.
Rinse fish with cold water and pat dry. Discard the marinade. In one dish, stir together the polenta, corn chip crumbs, salt and pepper. In another dish, stir together 2 tablespoons of lime juice and canola oil. Dip fillets into the lime and oil, then into the polenta mixture to coat. Place the fish onto the roasting rack.
Bake fish for 8 to 10 minutes, or until it flakes easily with a fork. While the fish is baking, combine the processed cheese, chilli powder, cumin and fresh chilli in a small saucepan over medium-low heat. Cook and stir until melted and smooth.
Place fish onto serving plates and spoon the cheese sauce over them. Top with a sprinkling of coriander leaves, if desired.