How to Make It
Heat 1/4 cup of the extra-virgin olive oil in a large saucepan. Add the onions, cover and cook over low heat, stirring occasionally, until softened but not browned, about 20 minutes. Add the bell peppers and water, cover and cook, stirring occasionally, until the vegetables are very tender, about 20 minutes. Season with salt and black pepper.
In a large skillet, heat 1/8 inch of the pure olive oil. Add half of the potato slices in a single layer and cook over moderately high heat until nicely browned on the bottom, about 5 minutes. Turn and cook until golden on the second side, about 3 minutes. Transfer the potato slices to a baking sheet lined with paper towels to drain. Repeat with the remaining potatoes, reducing the heat to moderately low if the oil gets too hot. Wipe out the skillet.
Preheat the oven to 450°. Add 1/8 inch of the pure olive oil to the skillet and heat. Season the bass fillets with salt and black pepper. Add 4 of the fillets to the skillet and brown them on 1 side over moderately high heat, about 3 minutes. Turn and cook until firm on the second side, about 1 minute; transfer the fillets to a large baking sheet. Repeat with the remaining fish.
Grease a large roasting pan with pure olive oil. Remove the paper towels from the baking sheet and bake the potatoes for about 5 minutes or until heated through. Season the potatoes with salt and black pepper. Arrange them, overlapping around the edge of the roasting pan.
Reheat the onions and bell peppers, then spread the mixture in a neat square inside the potato border in the roasting pan. Set the fish fillets on the onion mixture, browned side up, and bake for about 20 minutes, or until the fish is just cooked through, the potatoes are crisp and the onion mixture is bubbling.
Meanwhile, in a medium bowl, toss the fennel with the remaining 2 tablespoons of extra-virgin olive oil and the lemon juice. Season with salt and black pepper and stir in the sesame seeds.
Using a metal spatula, transfer the bass fillets and vegetables to plates. Top the fish with a spoonful of fennel salad and serve.