1. Cut the onion into small pieces and set aside in a bowl or small container.
2. Cut the bell peppers and set aside in another container.
3. Cut the beef into small pieces and set aside in a third container.
4. The goal is to add the ingredients one after the other in the wok.
5. Heat the wok over high heat with 2 or 3 tablespoons of oil. Add the sesame seeds.
6. When they are toasted (light brown to dark brown, according to your taste, it takes about 10 to 30 seconds), add the onion and stir vigorously. When they have started to brown (1 minute), add the bell pepper and mushrooms.
7. The bell pepper takes a little longer to cook. After a few minutes, add the beef and continue to stir vigorously.
8. If the wok starts to stick, occasionally add a little water to lower the temperature.
9. When the ingredients are sufficiently browned, reduce the heat and cover to continue cooking.
10. When it is cooked, add the cilantro to garnish.
11. Serve with rice.