Bring 1 1/2 cups water and 1 cup of the wine to a boil in a large, shallow pot. Add the mussels, cover the pot, and cook over medium heat for 5 minutes, until the mussels open. When the mussels are cool enough to handle, separate the mussels from the shells and set aside, discarding the shells and any mussels that didn't open. Strain the cooking liquid through a cheesecloth-lined sieve into a bowl and reserve.
Meanwhile, melt the butter in another large heavy-bottomed pot over medium heat. Add the onions, leek, carrots, garlic, saffron, 2 teaspoons salt, and 1 teaspoon pepper and saute, stirring occasionally, for 5 minutes. Reduce the heat to low, cover, and cook for 20 minutes. Add the tomatoes and cook for another minute.
Add 2 cups of the reserved cooking liquid (discard the rest) and the rest of the wine, bring to a boil, lower the heat, and simmer uncovered for 5 minutes. Add the reserved mussels, the half-and-half, and the cream. Heat until just heated through but not boiling. Stir in the parsley or dill, 1 teaspoon salt, and 1/2 teaspoon pepper. Serve hot.