In a large sauté pan add onion and garlic, cook until onions become translucent. Add shrimp, salt, cayenne, pepper, and oregano flakes. Cook until shrimp is barley pink, about 3-4 minutes. When shrimp is done, turn off burner and add cream cheese, parsley, and bread crumbs. Stir gently to combine.
Take fillet and lay down on cutting board on the edge of a table. With a sharp knife slice a pocket into the side of the fillet. When you insert the knife into the side do so at an angle so that the pocket inside of the fish is wider than the opening where you inserted the knife. This will help keep all the stuffing inside the filet while it is baking.
Rub top of fillet with olive oil and season with salt and pepper
Take shrimp mixture and carefully scoop it inside the pocket. Spread stuffing out evenly.
Place stuffed fillet in a large baking dish
Take halved tomatoes and season with salt and pepper, add any leftover onions and herbs, and toss in mixing bowl with olive oil, arrange around fillet and bake in oven for 12-15 min at 375°F. Top each serving with a slice of avocado.