In a medium saucepan, combine the vegetable broth, miso paste, soy sauce, sesame oil, ginger, garlic, and half of the green onions. Stir well to dissolve the miso and bring the mixture to a gentle simmer over medium heat.

Once simmering, carefully add the salmon fillets to the broth. Cover the saucepan and poach the salmon for about 10-12 minutes, or until it flakes easily with a fork.

While the salmon is cooking, add the bok choy and mushrooms to the broth during the last 5 minutes of cooking.

Once the salmon is cooked, carefully remove the fillets and set them aside. Stir in the rice vinegar and chili flakes (if using) into the broth. Season with salt and pepper to taste.

To serve, ladle the broth and vegetables into bowls, place a salmon fillet on top, and garnish with the reserved green onions.