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.