Preheat the oven to 450 degrees F. Toss the bell pepper halves with 1 tablespoon olive oil and 1/2 teaspoon salt in a large microwave-safe bowl. Cover and microwave until the peppers soften, 10 to 12 minutes.
Meanwhile, mix the ragu, couscous, raisins, 1/2 cup cheese, 3 teaspoons vinegar, the parsley and 1/4 teaspoon salt in a bowl.
Whisk the remaining 1 tablespoon olive oil and 1 teaspoon vinegar, the tomato paste and 3/4 cup water in the bottom of a large nonreactive baking dish. Carefully transfer the peppers, cut-side up, to the baking dish and fill with the couscous mixture. Sprinkle with the remaining 1/4 cup cheese.
Cover the dish with foil and bake until the peppers are tender and the filling is hot, about 20 minutes. Transfer to plates and drizzle with the cooking liquid from the dish.
Per serving: Calories 539; Fat 24 g (Saturated 8 g); Cholesterol 67 mg; Sodium 1,098 mg; Carbohydrate 60 g; Fiber 8 g; Protein 23 g