Preheat the oven to 400 degrees F.
For the black cod and ratatouille: Cover a baking sheet with a piece of parchment paper that is about 2-inches wider than the tray on each side (to wrap up the fish completely). Tightly line the vegetable slices to accommodate the length of the fish in an alternating pattern. Season the slices with salt and pepper. Lay the black cod on top of the vegetables. Drizzle with olive oil.
Garnish with the red pepper, garlic, and thyme. Season with salt and pepper. Add the butter on top.
Bring up each side of the parchment paper to the center and make small folds downwards, creating a secure bundle. Crimp the ends and twist to seal the fish and ratatouille inside.
Place in the oven and bake for about 20 minutes, until flaky.
For the herb salad: Mix the basil, chives, parsley, celery leaves, chervil, and dill together. Drizzle the vinegar and oil over top. Season with salt and pepper and toss!
For serving: Transfer the papillote to a serving plate. With a knife, carefully cut the paper to open the papillote and garnish with the herb salad.