Preheat oven to 350 degrees.
In the center of the parchment paper on a large baking tray, drizzle with olive oil and place the whole fish. Drizzle olive oil and fresh lemon juice on both sides of the fish as well as the inside cavity, and season well with salt and pepper.
Place the carrots, celery and quartered onions around the fish remembering to drizzle the
aromatics with olive oil and season with salt and pepper and top with the capers.
Bring the parchment paper together to form a pouch, tuck the tail of the parchment paper
under the fish or staple it shut. You want to make sure you have a tight seal.
Bake for approximately 40 minutes. The fish will be done when it is flaky & opaque.