1. Mix the Asian chili sauce, ginger, sugar or honey, oil, and tamari together in a large bowl. Set aside.
2. Use a damp paper towel to clean the portobello mushroom caps. Remove any discard any stems, then slice the mushroom caps into thin strips.
3. Heat olive oil in a large non stick pan over medium to medium high.  When the oil starts to shimmer, add the mushroom slices (1/3 of the batch at a time) and saute, flipping while cooking, for about 2 minutes or until the mushrooms are coated in oil and begin to soften. (They will shrink quite a bit.) Continue until you have cooked all the mushrooms.
4. Transfer the sauteed mushroom slices to the chili sauce bowl. Toss well to combine.

Assemble the burgers.

1. Lightly toast the millet and chia seed buns and then schmear one side with vegan sauce of choice (i.e hummus, vegan mayo, etc.).
2. Place lettuce on the bottom bun, followed by 3-5 slices of the marinated mushroom steaks, sesame seeds, sliced bell pepper, and any extra veggies and sauce (from the bowl) on top.  Serve immediately.