Prepare the miso salmon. Mix together miso paste with mirin and sake and brush on top of the salmon flesh. Bake for 12-14 minutes or until the fish flakes easily.
Meanwhile, make the mushroom rice. Melt 2 tablespoon butter in a medium saute-pan over medium heat. Add the mushrooms and saute until browned, about 5 minutes. Season the mushrooms with salt and pepper. Add the cooked rice, 2 tablespoon butter, and soy sauce, and stir until well combined.
Portion the rice into bowls and top with the salmon. Scoop a generous amount of ikura on top and garnish with scallions and nori. Serve immediately.