Begin by preparing a deeply flavored seafood broth enriched with shells and trimmings. In a tall pot, such as an 8-quart stockpot, warm the olive oil over medium heat. Add the chopped onion and leek and cook slowly, stirring occasionally, until they soften and become translucent without browning. Stir in the smashed garlic and cook just until fragrant, about a minute. Add the tomatoes, water, herb sprigs, orange peel if using, saffron, sea salt, and the fish heads, bones, and shrimp shells. Bring the mixture up to a gentle simmer, then lower the heat so the surface bubbles lazily rather than boiling vigorously.