Add 1/4 cup water to the bulgur wheat in a small bowl and let it sit for 60 minutes. The wheat will double in size.
Steam the quartered mushrooms for about 10 minutes, or until tender. Remove the mushrooms and put onion in steamer. Steam onions for 10 minutes, until translucent. Keep these two ingredients separate and set aside.
Add 1/2 cup water to the oats, and soak 10 minutes, until soft.
Drain excess water from bulgur and oats. Then combine grains with the steamed mushrooms, onions, rice, cheeses and spices in a food processor. Pulse 4 or 5 times to chop finely, but not puree. You want a coarse texture, with chunks of grain, mushroom and cheese that you can see.
Pour the mixture into a bowl with the steamed mushrooms and cornstarch. Mix well.
Preheat oven to 300°F and set a large skillet over medium low heat.
Spray the skillet with the cooking spray. Measure 1/2 cup at a time of the burger mixture and shape into a 4" patty, about 1/2" thick. Cook the patties about 2-4 minutes per side, until light brown.
When all the patties have been cooked, arrange them on a baking sheet and bake for 20 to 25 minutes, turning them halfway through cooking time. Serve immediately, or freeze.
If freezing, you can reheat several ways. Spread a light coating of olive oil spray on each side and heat in a pan over medium heat 3-4 minutes for each side, until hot in the middle. If using the grill, spray each side with oil and cook over low flame. Cook 3 to 4 minutes per side. You may also microwave for 30-35 seconds, then place in toaster and cook on medium heat till hot in the center. Enjoy!