Mix the miso paste, mirin, soy sauce, rice vinegar, sesame oil, and grated ginger in a bowl until smooth.
Pat the salmon fillets dry with paper towels and place them on a lined baking sheet. Brush generously with the miso mixture, reserving some for serving. Let marinate for 15-30 minutes.
Preheat your broiler to high. Position the rack about 6 inches from the heat source.
Broil the salmon for 8-10 minutes, or until the glaze is caramelized and the fish flakes easily with a fork.
While the salmon cooks, prepare your bowl bases by dividing the rice, spinach, avocado, carrots, and cucumber between four bowls.
Place the cooked salmon on top of each bowl, brush with remaining glaze, and garnish with sesame seeds and nori strips if using.