Baking fresh ingredients in parchment packets requires less fat than other cooking methods and keeps fish moist and tender. Quick cleanup is an added bonus.
Preheat oven to 400 degrees. Divide bok choy among four 16-inch-long pieces of parchment paper. Top with scallions and mushrooms. Season with salt and pepper. Top each with 1 lemon slice, 1 bass fillet, and another lemon slice and drizzle with 1 1/2 teaspoons oil. Season with salt and pepper. Bring long sides of paper together and fold down 3 times to make a seam. Tuck ends underneath to secure.
Place packets on a rimmed baking sheet and bake until fish is opaque throughout and vegetables are tender, 15 minutes. Carefully cut open each packet and serve immediately.