1) Preheat oven to 180 degrees C. Soak 4 small brown paper bags in vegetable oil until saturated. Scrape off excess oil and set aside.
2) Season the fillets with salt, grated ginger, lemon zest and juice. Top each with about 5 or 6 mangetout and 1 of the thinly sliced potatoes.
3) Transfer each fillet into 1 of the bags and pour about 60ml of the coconut milk into each bag. Fold well to seal in juices and steam. Place onto 2 rimmed baking trays and bake until bags are puffed and fish and potatoes are tender, about 10 to 15 minutes.
4) Serve immediately at the table in the puffed bags. Tear open.