Step 1 Preheat oven to 140°C. Toss steak in flour to coat, shaking off any excess. Spray a large flameproof casserole dish with olive oil and set over high heat on the stovetop. Add steak in batches; cook for 2–3 minutes, or until browned. Remove beef from dish and set aside.
Step 2 Spray dish with oil again; set over medium heat. Add onion, carrots and celery; cook, stirring occasionally, for 5 minutes, or until soft. Add garlic and paprika; stir for 1 minute, or until fragrant.
Step 3 Add tomato paste to dish; stir for 1 minute. Add vinegar; simmer for 1 minute. Add tomatoes and stock with 1/2 cup water; bring to the boil. Return reserved beef to dish, cover and transfer to oven; cook for 2 1/2 hours, or until meat is extremely tender. Remove dish from oven. Shred meat with a fork and stir silverbeet through beef ragu until just wilted.
Step 4 Meanwhile, cook rigatoni in a large saucepan of boiling water according to packet instructions until al dente; drain and return to pan. Add beef ragu; toss. Divide mixture among 4 bowls, garnish with parsley and serve.