Prepare croutons. Preheat oven to 375°F. Slice the baguette on a ½-inch thick bias cut. Lightly brush each side with olive oil and sprinkle salt. Place on a sheet pan and bake for 10 minutes. Flip over and cook another 2 to 5 minutes until golden brown on each side. Cool and reserve.
In a large pot, heat the oil over medium heat until hot.
Add the onion and garlic and cook until translucent, about 5 minutes.
Add the leeks, celery, bell pepper, carrots, and minced serrano chili peppers. Cook for another 5 minutes.
Add the tomatoes and cook 3 minutes.
Add the tomato paste, wine, water, lemon juice, thyme, bay leaf, ½ teaspoon salt, and ¼ teaspoon pepper. Simmer on medium-low heat for 25 minutes, stirring occasionally. Remove the bay leaf.
Clean and remove the beard of the mussels. Discard any mussels that have broken shells or do not close when tapped.
Heat the cioppino soup over medium heat. Add shrimp and scallops to the soup, cover and cook for 2 minutes.
Open the lid and flip over the shrimp. Insert the mussels evenly spread across the pan. Turn heat to medium-high, cover and cook another 3 to 5 minutes until the shrimp is cooked and mussels are open. Discard any mussels that do not open.
Taste soup and season with more salt and pepper as needed.
To Serve: Ladle soup into serving bowls, add seafood, and garnish with chopped parsley and more thyme. Serve with homemade crouton.
If you do not want a slight spiciness to the stew, omit the serrano peppers.