Preheat the oven to 350 degrees.

Cut the fish into smaller pieces and use a paper towel to pat the fish dry .

Heat about 1 tablespoon of butter in a nonstick skillet over medium-high heat.

Season both sides of the fish with 1 teaspoon salt and 1/2 teaspoon of the pepper and cook the fish for about 2-3 minutes per side. It should still be partly raw in the center. Cook the fish in batches until it's all partly cooked, transferring to a plate.

In the same skillet, heat 1 tablespoon of butter on medium high heat and add the onions to the skillet, seasoning with half of the remaining salt and pepper. Cook the onions for 5 minutes, until they are translucent and soft.

Add the carrots, celery, bell pepper, tomatoes and garlic. Season with remaining salt and pepper as well. Cook for another 5-7 minutes.

Remove from heat and stir in the fresh herbs.

Spread a thin layer of vegetables and herbs on the bottom of a large, deep baking dish.

Place one layer of fish over the vegetables. Spread half of the remaining vegetables over the fish.

Add the remaining fish on top of the vegetables and layer with the last of the vegetables.

Spread a thin layer of mayonnaise over the vegetables. Cover the baking dish with a lid or aluminum foil and bake in the preheated oven for about 30-40 minutes.