Step 1 Preheat oven to 160°C. Spray a large deep flameproof, ovenproof casserole dish with olive oil; set over high heat on the stovetop. Add steak and cook in 2 batches, stirring, for 2–3 minutes, or until golden brown. Remove beef from dish and set aside.
Step 2 Return dish to medium heat; spray with olive oil again. Add onion, celery and carrot to dish; cook, stirring occasionally, for 5 minutes, or until vegies are soft. Add garlic to dish; cook, stirring, for another 30 seconds. Add red wine to dish; simmer until mixture reduces by half.
Step 3 Add tomato paste to dish and cook, stirring, for 1 minute. Add chopped tomatoes to dish with 3/4 cup water. Transfer reserved beef to dish and bring to the boil; cover dish and transfer to oven. Bake for 1 1/4 hours, or until beef is almost tender. Add sweet potato and zucchini to dish, and bake for another 20 minutes, or until beef and vegies are tender. Season with cracked black pepper.
Step 4 Increase oven temperature to 190°C. Transfer beef mixture to an 8-cup-capacity ovenproof pie dish or baking dish.
Step 5 Meanwhile, bring 3 cups water to the boil in a large saucepan. Reduce heat to low. Add instant polenta in a slow, steady stream; cook, stirring, for 6–7 minutes, or until polenta is thick and creamy.
Step 6 Spread polenta carefully over beef mixture to cover. Bake for 20 minutes, or until golden. Set dish aside for 10 minutes to cool slightly, and serve.