Saffron adds its characteristic bittersweet earthiness to the broth, though the dish is good without it, too. Serve with toasted slices of French bread.
Heat oil in a large saucepan over medium-high heat. Add onion to pan; sauté 1 minute. Add garlic; sauté 30 seconds. Add saffron; sauté 15 seconds. Stir in tomato paste, whipping cream, and clam juice; bring to a boil. Cook for 1 minute, stirring occasionally. Add mussels to pan. Cover and cook 5 minutes or until mussels open; discard any unopened shells.