Preheat the grill in your oven to the highest heat and place a rack on the top shelf of the oven.
Mix the miso, mirin, lemon juice, garlic and honey together. Place the salmon fillets skin side up onto a baking tray lined with some lightly greased foil. Place under the grill for 3 minutes then flip the fillets so they are skin side down. Cover with the miso glaze and return to the oven to grill for 2-3 minutes more.
Boil the kettle with a mugful of water. Place the spinach into a large colander and place into the sink. Pour the boiling water over the spinach to wilt it. Place into a bowl and toss with the ginger, sesame oil and season with salt to taste.
Serve the salmon on top of the spinach and top with the sesame seeds.