In a medium saucepan, combine the water, miso paste, soy sauce, rice vinegar, sesame oil, ginger, and garlic. Whisk until the miso is fully dissolved and the mixture is well combined.

Bring the broth to a gentle simmer over medium heat.

Carefully add the salmon fillets to the broth, skin-side down. Reduce the heat to low, cover, and poach the salmon for about 10-12 minutes, or until the salmon is opaque and flakes easily with a fork.

In the last 2 minutes of cooking, add the sliced mushrooms and baby spinach to the broth to wilt the spinach and soften the mushrooms.

Once cooked, carefully remove the salmon fillets from the broth and place them on serving plates.

Ladle the broth with mushrooms and spinach over the salmon, and garnish with sliced green onions and sesame seeds if desired.