Heat the oil in a very large pot over medium heat. Add the onion and garlic, and saute until tender, about 8 minutes. Add 1/3 of the spinach and saute until it begins to wilt, about 1 to 2 minutes. Continue adding the spinach 1 handful at a time, sauteing just until it begins to wilt before adding more. Add the soy sauce and crushed red pepper. Saute over high heat until spinach wilts. Season the mixture, to taste, with salt and black pepper. Transfer the mixture to a bowl and serve.