How to Make It
In a saucepan of boiling water, cook the spinach until just wilted, 30 seconds. Drain and spread on a baking sheet to cool, then gently squeeze out the excess water.
In a large pot, bring the wine and water to a boil with the bay leaves. Add the mussels, cover and cook over high heat, shaking the pot a few times, until the mussels open, about 5 minutes. Transfer the mussels to a large, rimmed baking sheet to cool. Carefully pour the mussel cooking liquid into a glass measuring cup, stopping before you reach the grit at the bottom; discard the bay leaves. Remove the mussels from their shells.
In a large saucepan, melt the butter. Add the shallots and cook over moderate heat until softened, about 5 minutes. Stir in the flour. Gradually whisk in the reserved mussel cooking liquid and the clam juice. Bring to a boil, whisking. Whisk in the tomato paste and the saffron and simmer over low heat, whisking occasionally, until no floury taste remains, about 10 minutes. Stir in the milk and cream and simmer for 5 minutes. Stir in the spinach and mussels and season with salt and pepper. Ladle into bowls and serve.