Start the Teriyaki Sauce: In a small saucepan, whisk together tamari, orange juice, maple syrup, brown rice vinegar, ginger powder, and garlic powder. Bring to a simmer over medium heat.
Add a cornstarch slurry: In a separate small bowl, whisk together cornstarch and water to make a slurry. Pour into the saucepan with the teriyaki sauce and whisk to combine. Simmer for 1-3 minutes, until the sauce thickens. Remove from the heat and let cool.
Marinate the veggies: once your teriyaki sauce has cooled, add the prepared veggies (NOT the pineapple) to a large ziploc bag. Pour in ½ cup of the teriyaki sauce, seal the bag, and gently move the veggies around until evenly coated. Lay the bag flat in the refrigerator to marinate for at least 30 minutes. Reserve the remaining teriyaki sauce for later.
Assemble the kebabs: Thread the marinated vegetables and pineapple chunks onto metal skewers in whatever pattern you like. Briefly dip each pineapple chunk into the remaining teriyaki sauce before adding it to the skewer. This should make about 10 skewers.
You may grill the kebabs, OR roast them in the oven!To cook on the grill: Preheat the grill to medium heat and spray with a bit of oil to prevent sticking (optional). Place the kebabs on the grill and cook for 10-15 minutes, turning every 3-5 minutes, until lightly charred and tender. During the last few turns, brush the kebabs with additional teriyaki sauce. To roast in the oven: Preheat oven to 400F and line a baking sheet with a silicone baking mat or parchment paper. Arrange the kebabs in a single layer and bake 18-22 minutes, turning and brushing with extra teriyaki sauce halfway through. Serve these as a side, or over rice with extra teriyaki sauce drizzled on top!