Pre-heat oven to 425F degrees. Line a large sheet pan with four good-sized pieces of aluminum foil. Set aside
Slice the potatoes to about 1/4 inch thick. Dip the potato slices into a bowl with melted butter, then arrange the potato slices on the foil overlapping each other, forming a bed of potatoes. Season the potato slices with sea salt and top with fresh thyme. Repeat until all four pieces of foil have a layer of potatoes. Bake for 20-25 minutes.
Meanwhile prepare the cod by cutting it into four equal size pieces. Brush with remaining butter and season with sea salt, freshly ground pepper and fresh tyme. Top with lemon slices.
Place a piece of cod on top of each potato bed. Gently fold the sides of the foil up, not completely wrapping the fish but rather forming a pocket. Bake for 18-20 minutes or until fish is flaky.
Serve each fish packet with fresh lemon slices, fresh herbs and a side of green salad