Heat oven to 400 degrees. Spray a large baking sheet with olive oil.
Spread cornmeal on a large plate. Mix in salt and pepper. Dip fish lightly into the cornmeal. Arrange evenly on the baking sheet. Spray the top of the fish with olive oil then place in the oven to bake 20 minutes, until it’s cooked through and flakes easily with a fork.
Mash avocado in a bowl. Mix in red onion, lime juice, and salt.
Serve fish topped with a large dollop of avocado puree.