How to Make It
In a nonreactive medium saucepan, toast the fennel seeds over moderately high heat, shaking the pan, until fragrant, about 2 minutes. Add the orange juice, sliced fennel and saffron and bring to a boil. Lower the heat to moderate and simmer until the juice has reduced to 1 cup, about 20 minutes. Strain the juice into a bowl, pressing down on the cooked fennel; reserve the fennel.
Return the juice to the saucepan and bring to a simmer over moderate heat. In a small bowl, mix the arrowroot with 2 tablespoons of water, then whisk into the juice and cook just until thickened, about 30 seconds. Remove from the heat, stir in the cooked fennel and let stand at room temperature until cool. Strain the mixture into a medium bowl and whisk in the olive oil, lemon juice and mustard. Season the dressing with salt and white pepper. Stir in the basil just before serving.