Preheat oven to 190°C (375°F).

Cook pasta in a saucepan of boiling salted water for about 3 minutes. Add broccoli to pasta and cook 1 more minute. Drain and set aside.

In a skillet, fry the onion in oil over medium-high; add the beef and fry until meat is no longer pink. Remove from heat and add Alfredo sauce; mix well. In a large bowl, combine the pasta, broccoli, and sauce. Season with salt and pepper. Transfer mixture into a 33 x 22cm (13 x 9 in.) baking dish and top with cheese. 

Bake until cheese has melted, about 8 minutes, and finish under the broiler for about 2 minutes, or until cheese is golden brown.