For the Aioli: Place the pasteurized egg yolk, lemon juice, cold water, mustard, and ½ teaspoon of salt in a medium-size nonreactive bowl. Whisk vigorously until blended. Very slowly, drop by drop, whisk in about a quarter of the olive oil, whisking all the while. This will take several minutes; go slowly, allowing the mayonnaise to thicken. Gradually add the remaining olive oil until the desired thickness is reached; you might not use all the oil, but you will most likely use at least ¾ cup (180 ml). Gently stir in the chopped peppers and cayenne to taste.

Season to taste with more salt and with black pepper, if desired. The aioli is ready to use, or refrigerate in an airtight container for up to 2 days.

For the Toast: Right before beginning to cook the seafood, slice the baguette into 1-inch (2.5 cm) slices, toast in a toaster or on a baking sheet in a 400°F (200°C) oven, then brush with the olive oil; set aside.

For the Bouillabaisse: Peel the shrimp and set them aside, reserving shells. Place the shells in a medium-size saucepan with 4½ cups (1 L) of water. Bring to a boil, lower the heat, and simmer for 5 minutes. This is a super-quick shellfish stock.

While the shrimp shells are simmering, heat the oil in a deep 5-quart (4.7 L) pot, such as a Dutch oven, over medium heat. Add the leek greens and sliced fennel and sauté for 3 to 5 minutes, or until softened but not browned.

Strain and measure out 4 cups (960 ml) of the stock for the bouillabaisse. Discard the shells. Add the shrimp stock, tomatoes, parsley, orange zest, thyme, and fennel seeds to the leek mixture. Cover, increase the heat, and simmer for 10 minutes

Add the clams and/or mussels, cover, and cook for about 5 minutes, or until the shells open, discarding any that do not open. Add the white fish and scallops, cover, and cook for about 5 minutes, or until the fish is almost opaque. Add the shrimp and cook, covered, for about 3 to 5 minutes more, or just until the shrimp turn pink. Gently stir in the basil and taste the broth. Season with salt and pepper, as desired.

Place a slice or two of garlic toast in each bowl and top the toasts with a generous dollop of aioli. Ladle the stew on top and garnish with fennel fronds, if you like. Serve immediately.