How to Make It
In a small saucepan, bring 1/2 cup of the fish stock to a simmer. Transfer to a measuring cup and crumble in the saffron.
In a large skillet, heat 2 tablespoons of the oil. Add the garlic and cook over moderate heat until fragrant, about 2 minutes. Add the mussels, cover and cook until they start to open, about 4 minutes. Add 1/2 cup of the wine and simmer over moderate heat until reduced by half, about 4 minutes. Add the saffron-infused fish stock and the parsley and simmer for 3 minutes; discard any mussels that don't open. Transfer 16 mussels to a bowl. Remove the remaining mussels from their shells and put them in another bowl with their cooking liquid.
Bring the remaining 5 1/2 cups of fish stock to a simmer over low heat. In a large saucepan, heat the remaining 2 tablespoons of olive oil. Add the shallot and cook over moderate heat until softened, about 4 minutes. Add the rice and cook, stirring, until thoroughly coated with oil, about 3 minutes. Add the remaining 1/2 cup of wine to the rice and simmer until reduced by half.
Add enough hot stock to the saucepan to just cover the rice and cook over moderate heat, stirring, until the stock has been absorbed. Repeat with the remaining stock, adding just enough each time to cover the rice and stirring until it has been absorbed before adding more. The rice is done when it's just tender and bound with a creamy sauce, about 20 minutes total. Stir in the butter and the shelled mussels and their cooking liquid. Season with salt. Divide the rice among 4 bowls and top each with a few of the mussels in their shells and the bottarga. Serve right away.