Before starting, check how long your rice or noodles will need to cook and plan this step accordingly.
In a mixing bowl, combine 2 tablespoons of the tamari/soy sauce, 1 tablespoon of the mirin, and 1 tablespoon of sesame oil. Chop the tempeh into even, bite-sized pieces and add these to the bowl. Toss well to coat each piece evenly, then marinate for at least 15 minutes (or cover and marinate in the fridge for up to 24 hours). You can use this time to prepare the other ingredients.
Heat a large frying pan or wok on medium heat. Spoon the tempeh pieces into the pan, leaving some of the marinade in the bowl. Fry gently for 2-3 minutes on each side or until lightly browned. The sesame oil in the marinade should mean you don't need cooking oil here, but drizzle some in if the tempeh is sticking.
While the tempeh is cooking, prepare the teriyaki sauce. To the bowl with the leftover marinade, add the other 2 tablespoons of tamari/soy sauce, the other 1 tablespoon of mirin, agave syrup, minced garlic, minced ginger, cornflour, and water. Mix well to make sure there are no lumps of cornflour.
Once the tempeh is cooked, remove from the pan and set aside. Using the same pan, heat the cooking oil on medium/high heat and add the mixed vegetables. Stir/toss frequently for 6-7 minutes or until the veg is starting to colour and become tender.
Once the veg has cooked, reduce the heat to medium and add the tempeh pieces and teriyaki sauce (giving this a quick mix again beforehand). Mix everything thoroughly and cook for a couple of minutes, stirring in extra water if it's looking too thick.
Serve over your choice of rice, rice noodles, or other sides. Top with sliced spring onions and sesame seeds.