Cut the beef, potatoes, and carrots into small pieces, and cut the onion into quarters. Place the beef and vegetables in a roasting bag. Add seasoning to taste and greens to taste. Pour in 200 ml water or broth. Close the bag securely and distribute the mixture evenly inside.
Place the bag in a baking dish. Bake in a preheated oven at 160°C (320°F) on the lower shelf for 90 minutes.
Carefully cut open the top of the bag with scissors—be careful of the hot steam. Bake the roast uncovered at 200°C (400°F) for 15 minutes.