For the saikyoyaki marinade: Mix all ingredients to a smooth paste.

For the lamb: Sprinkle the lamb loin with a pinch of salt and let rest in the fridge for 30 minutes. Rinse the lamb loin with sake and pat dry with paper towel. Reserve 100g of marinade to finish the dish, and marinate the lamb loin in the rest of the marinade for 1 day.
Remove the lamb loin from the marinade and wipe dry with paper towel. Roast the lamb loin in a pre-heated oven at 200°C for 8 minutes or until medium rare. Remove from the oven and rest the lamb in a warm place.
For the spinach: Sauté washed baby spinach in a fry pan with oil and butter. Season with salt and pepper and finally, strain excess oil from the vegetables.

Serving
Arrange sautéed spinach in the center of a plate. Using a sharp knife, slice the lamb loin into 6 pieces per portion. Fan them in an orderly manner beside the sautéed spinach. Lace the lamb pieces with the saikyoyaki marinade. Garnish with kaiware.