Bring a medium saucepan of salted water to a boil. Add the couscous and cook as the label directs. Drain and set aside.
Meanwhile, heat the olive oil in a large pot or Dutch oven over medium-high heat. Add the onion, bay leaf and a pinch of salt. Cook, stirring occasionally, until the onion is softened, about 6 minutes. Add the garlic and red pepper flakes and cook, stirring, until the garlic is golden, about 3 minutes. Add the wine and cook until reduced by half, about 4 minutes. Add the tomatoes and cook, stirring occasionally, until thickened, about 10 minutes.
Add 1 1/2 cups water, the mussels and 1/2 teaspoon salt. Cover and cook until the mussels open, about 5 minutes (discard any that do not open). Remove with a slotted spoon and divide among bowls. Add the couscous and spinach to the liquid in the pot and heat through, about 1 minute; season with salt. Add the couscous mixture to the bowls and sprinkle with the basil.