How to Make It
In a glass measuring cup, warm the wine in a microwave oven. Lightly crumble the saffron into the wine. Heat the olive oil in a large, deep skillet. Season the chicken breasts with salt and pepper and add them to the skillet, skin side down. Cook the chicken over moderately high heat for 2 minutes. Reduce the heat to moderate and cook until the skin is browned and crisp, about 4 minutes. Turn and cook for 3 minutes longer. Transfer to a plate.
Melt the butter in the skillet. Add the leeks and cook over moderate heat, stirring, until softened, about 3 minutes. Add the garlic and cook over moderately low heat until golden, about 3 minutes. Add the wine and saffron and simmer for 2 minutes. Add the stock and bay leaf and bring to a simmer over moderately high heat. Return the chicken breasts to the skillet, skin side up and simmer over moderately low heat until just cooked through, about 10 minutes. Transfer the chicken to a warmed platter.
Add the cream to the skillet and simmer over moderately high heat until slightly thickened, about 4 minutes. Add the spinach and cook until wilted, about 1 minute. Remove the skillet from the heat and stir in the mustard; season the sauce with salt and pepper. Discard the bay leaf. Spoon the sauce onto plates, top with the chicken breasts and serve.