Bring 4 cups water, celery, soy sauce, onion powder, garlic powder, and pepper to a boil in large pot over high heat. Reduce heat to medium and simmer 5 minutes. Stir in oats, chopped mushrooms, and flour, and cook 5 minutes more. Transfer to bowl and chill.
Preheat oven to 350 degrees. While mixture is chilling, heat olive oil in large pan over medium heat. Add onions and cook until almost golden, about 5 minutes. Add sliced mushrooms and saute 5 minutes more, until mushrooms are browned and onions are caramelized.
Spray large loaf pan with cooking spray. Add half of meatloaf mixture, top with ½ cup cheese, and add the rest of the meatloaf mixture on top of this. Top the meatloaf with sauteed onions and mushrooms and pour pasta sauce on top of this. Sprinkle with 2 tbsp. of cheese. Cook for 1 hour or until set.