1. Heat the oil in a large skillet. Add the chorizo and cook over moderately high heat, stirring frequently, until browned, about 10 minutes. Transfer to a plate. Pour off all but 1 tablespoon of the fat.
2. Add the chopped onion to the skillet and cook over moderately low heat, stirring occasionally, until translucent, about 5 minutes. Add the wine and minced garlic and cook until the liquid is reduced to 1/4 cup, about 3 minutes. Add the chorizo, clam juice, crushed tomatoes and thyme and bring to a simmer. Cover and cook for 20 minutes.
3. Meanwhile, cut a 3-inch piece from the baguette and cut off the crust. Soften the piece of crustless bread under running water, then squeeze to remove the excess water. In a food processor, puree the softened bread with the mayonnaise, red peppers, smashed garlic cloves and cayenne.
4. Preheat the broiler. Cut the remaining baguette into 1/4-inch slices and arrange on a baking sheet. Broil for about 3 minutes, turning once, until golden.
5. Add the shrimp and scallops to the skillet and simmer until just cooked through. Stir in 1/2 cup of the rouille and the black pepper. Season the stew and the remaining rouille with salt. Spread the rouille on the croutons. Serve the stew in shallow bowls, garnished with the croutons.
Wine Recommendation: The rich, earthy rosés of Provence go particularly well with shellfish stews. Look for the 1996 Domaine Jean Bagnis l'Estandon Rosé.