Preheat the oven to 200 degrees C/400 F and line a baking tray with parchment paper. Add the broccoli to the baking tray and drizzle with olive oil. Bake in the preheated oven for 10 minutes, until lightly crispy.
When the broccoli is almost done, heat the sesame oil in a skillet or wok over a medium-high heat. Add the mushrooms and red onion, cooking for 2-3 minutes until they begin to soften.
Add the broccoli and cook for a further 3 minutes to allow the florets to soften further.
Lower the heat to medium. Add the tamari, orange juice, rice vinegar, and water. Stir together for a minute or so, then add the cornstarch and cook for a further few minutes to allow the sauce to thicken.
At the last minute, stir in the sesame seeds and serve.
Do not throw away the broccoli stem: you can use it in other recipes, such as salads.