Cover a sheet tray with a large piece of parchment paper, leaving a little hanging over the edge.
Lay the fish on the parchment, season with salt and pepper, and place the butter on top. Lay lemon slices over the fish, then arrange the bok choy, radish, scallions and parsley around the fish. Staple a second piece of parchment to the first, covering the sheet tray entirely. Place in the oven and bake for 12 to 15 minutes.