This corn and tofu stir-fry is best done in a cast iron skillet, so that the tofu can develop a nice crispy crust. Slideshow: More Stir-Fry Recipes
Drain the tofu and slice into 1/2-inch-thick slices. Pat slices dry with paper towels. Season both sides with salt and pepper.
Heat a large skillet over heat. Add 2 tablespoons of oil, and then carefully add the tofu (hot oil may splatter). Sear both sides of the tofu until golden brown, 2-3 minutes each side. Remove the tofu from the pan and drain on paper towel towels.
Turn the heat down to medium heat. Add the final tablespoon of oil to the pan. Stir in the onions and cook until caramelized and golden brown, about 10 minutes, stirring occasionally.
Stir in the hoisin and fish sauce. Stir in the corn. Cook for 2-3 minutes or until the corn is tender.
Layer several pieces of tofu on each plate. Add a couple spoonfuls of corn mixture over tofu. Garnish with cilantro and drizzle with soy sauce if desired. Serve warm.