Wash the cod fillets and pat dry. Halve the lemon. Squeeze the juice from one half over the fish. Cut the other half into slices and set aside. Season with salt and pepper. Place the fish in a lightly oiled baking dish.
Heat a non-stick pan over medium heat. Add the pine nuts and toast for 2-3 minutes, stirring often, until golden brown. Transfer to a plate and cool for 1 minute. Roughly chop the cooled pine nuts. Chop the wild garlic. In a bowl, mix the pine nuts, wild garlic, and breadcrumbs. In a small bowl, combine the oil and mustard, then stir this mixture into the pine nut and breadcrumb mixture until well combined.
Spread the wild garlic and pine nut crust evenly over each fish fillet. Bake in a preheated oven at 200°C (400°F) for 20 minutes, until the fish is cooked through and the crust is golden brown. Serve with the lemon slices.