How to Make It
In a food processor, combine the egg, egg yolks, lemon juice and whole garlic cloves and pulse until the garlic is finely chopped. With the machine on, gradually add 3/4 cup of the olive oil until emulsified. Season the aioli with salt and black pepper and scrape into a bowl.
In a small saucepan of lightly salted boiling water, cook the potatoes until just tender, about 10 minutes. Drain and let cool, then slice 1/4 inch thick.
In a large cast-iron casserole, heat the remaining 1 tablespoon of oil. Add the onion and fennel and cook over moderate heat, stirring occasionally, until softened, 8 to 10 minutes. Add the sliced garlic and cook, stirring, for 1 minute. Stir in the thyme sprigs, bay leaves, orange zest, crushed red pepper and stock and simmer for 10 minutes. Season with salt and black pepper.
Add the fish fillets and potatoes to the casserole and simmer just until the fish is cooked through, 5 to 7 minutes. Discard the bay leaves, thyme sprigs and orange zest. Transfer the fish, broth and vegetables to shallow bowls and garnish with croutons. Drizzle with the aioli and serve with crusty bread.