Place peas in a bowl and cover with boiling water. Stand for 1 minute, then drain. Set 1 cup (120g) peas aside. Puree the remaining peas in a blender, then transfer to a saucepan over low heat. Season and keep warm.
Toss the fennel and reserved fronds, radish, celery leaves, onion and reserved 1 cup (120g) peas in a bowl. Combine the lemon juice and 1/4 cup (60ml) oil in a bowl, then season and toss with the salad. Set aside.
Season the salmon with salt. Heat remaining 1 tablespoon oil in a frypan and cook salmon, skin-side down, for 2-3 minutes until crisp. Turn and cook for a further 1 minute. Transfer to a plate and rest, loosely covered with foil, for 2 minutes. Serve salmon with the pea mash and fennel salad.