Preheat oven to 375 degrees F. Line a baking sheet with parchment paper.
Warm a drizzle of oil in a saute pan set over medium heat. Add onion and saute until onion is caramelized, about 5 minutes. Scrape into the bowl of a food processor. Refresh pan with oil. Add cremini mushrooms and saute until their liquid releases and evaporates, about 5 minutes. Add to the food processor along with beans, millet, miso, molasses, and sesame oil. Pulse until nearly pureed. Season with salt and pepper to taste.
Transfer contents to a mixing bowl. Fold in portobello mushroom, parsley, quinoa flakes, cornmeal, garbanzo flour, potato starch, and spices. Check for seasoning. Divide the mixture into 6 lumps. With moistened hands, form the lumps into patties. Space evenly on the prepared baking sheet. A ring mold would also work well to give the patties better shape. Bake the patties for 30 minutes, flipping halfway through. Let the burgers cool completely. At this point the burgers can be frozen.
To grill the burgers: Heat a grill pan or grill at medium-high heat. Brush the burgers with oil. Sear until heated through, about 4 minutes per side. Serve on buns with the fixings of your choice.