In a large pot, heat the oil over moderately high heat. Add the chorizo and cook, stirring frequently, until browned, about 10 minutes. Remove. Pour off all but 1 tablespoon fat.
Reduce the heat to moderately low. Add the onion and cook, stirring occasionally, until translucent, about 5 minutes. Add the wine and the minced garlic, bring to a simmer, and cook until reduced to about 1/4 cup, about 3 minutes. Add the cooked chorizo, the clam juice, tomatoes, thyme, and 1 teaspoon of the salt. Cover, bring to a simmer, and let simmer for 20 minutes.
Meanwhile, cut a 3-inch piece from the baguette and remove the crust. Hold this bread under running water to soften; squeeze to remove the water. Put this bread in a food processor. Add the mayonnaise, red peppers, cayenne, and the remaining garlic and 1/2 teaspoon salt and puree.
Light the broiler. Cut the remaining baguette into 1/4-inch slices and broil the bread on both sides until golden, about 3 minutes.
Add the shellfish to the stew; bring to a simmer, by which time they should be just done. Stir in 1/2 cup of the rouille and the black pepper. Serve with the remaining rouille and the croûtes.
Wine Recommendation: The rich, earthy rosés of Provence go particularly well with shellfish stews. Look for a wine from the commune of Bandol, especially from the great Domaines Ott.