1. Bring 8 cups of the water with 1 teaspoon of salt to a boil in a large pot. Add the basmati rice and cook uncovered on high heat for 10 minutes. Drain the rice well, return the rice to the same pot, and cover to keep warm.
2. Meanwhile, add the mussels to a medium skillet with 1 cup water, bay leaf, lemons, and whole peppercorns. Cook covered for 5 to 7 minutes on medium-high heat, until all shells are open.
3. Discard any mussels that do not open and transfer the open ones to a bowl. Strain the cooking broth and reserve. Squeeze the cooked lemons into the broth.
4. Heat the olive oil in a medium skillet. Add the shallots and sauté over medium-low heat for 5 minutes, then add the garlic and sauté for 30 seconds. Add the reserved broth, the tomato paste, the raisins, and 1/2 teaspoon salt. Stir until the tomato paste dissolves. Simmer rapidly, uncovered, for about 10 minutes to reduce and concentrate the sauce.
5. Divide the rice and mussels onto plates, pour the sauce over the mussels, and sprinkle with the walnuts.