For the seafood stock: Heat the oil in a stockpot over medium heat. Add the carrots, celery, fennel, garlic, leeks and onions and saute for about 5 minutes. Add the ketchup, rosemary, thyme, and the crab and lobster shells and stir. Add the peppercorns and 16 cups (4 liters) water. Bring to a boil, then lower the heat and simmer for 2 hours.
Strain the stock into a smaller saucepan. Simmer over medium heat until reduced by two-thirds, about 45 minutes. Stir in the butter and season with salt and pepper.
For the roasted leeks and mushrooms: Heat the oil in a saucepan over medium heat. Add the leeks and saute until golden brown, about 10 minutes. Add the mushrooms and cook until browned, about 5 minutes more. Add the roasted peppers (along with any of their juices) and a ladle of seafood stock. Season with salt and pepper. Keep warm.
For the rouille croutons: Soak 2 thick slices bread in the seafood stock. Squeeze out the liquid and add the bread to the food processor along with the garlic, chile and egg yolk. Buzz it up. Slowly add the olive oil while processing until a smooth paste forms. Add a little of the seafood stock if the rouille begins to split. Season with salt and pepper.
Cook's Note: Food Network Kitchens suggest caution in consuming raw and lightly-cooked eggs due to the risk of Salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly-refrigerated, clean, grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell. For recipes that call for eggs that are raw or undercooked when the dish is served use shell eggs that have been treated to destroy Salmonella, by pasteurization or another approved method.
Toast the remaining 4 to 6 slices bread (one for each serving) in a pan with olive oil over medium-high heat. Set aside.
For the seafood stew: Return the seafood stock to a simmer and add the clams. Cook until they start to open, about 5 minutes. Add the mussels and cook until they open, another 2 minutes. Add the shrimp and lobster meat and simmer for 2 minutes.
Before serving, add the peas to the leeks-mushroom mixture to warm them up. Spread the rouille on the toasted bread. Divide the leek-mushroom mixture among bowls, top with the crab meat, and then the seafood stew. Drizzle with olive oil. Garnish with a live scallop on the shell and sprinkle with chervil. Serve the rouille croutons on the side.
Warning: Consumption of raw or undercooked shellfish may substantially increase the risk of foodborne illness.