Cut the tempeh into small cubes, the pepper into bite-sized pieces, the onion into thin wedges.
Scatter these into a baking dish or an air fryer basket.
In a small bowl, mix the sesame oil, teriyaki sauce, vinegar, water and soy sauce. Taste and add a pinch of brown sugar if it’s too acidic.
Pour this mixture over the tempeh and the veggies, then toss gently.
Bake in a preheated oven for 25 minutes, at 375F/180C. If you’re using an air fryer, set it to "air fry" for 15 minutes at 180C.
Serve with the sesame seeds.