Whisk together the pineapple juice with cornstarch in a small saucepan over medium heat, and stir in the dry mustard powder, red currant jelly, raisins, and pineapple. (Lumps of jelly will dissolve as mixture heats.) Bring the sauce to a boil, stirring constantly, and simmer until thickened, 1 to 2 minutes. Keep raisins stirred into sauce as it cooks so they don't burn. Serve warm.
