Heat sesame oil in a large skillet over medium-high heat. Add cubed tofu and cook for 5 minutes, flip tofu and cook another 5 minutes until browned on the edges. Remove from skillet and set aside.
Add the red onion to the skillet, and sauté for 7 minutes until soft. Add mushrooms and sauté for 5 minutes until mushrooms release their juices. Stir in garlic and snap peas. Stir for about one minute.
Transfer the cooked tofu to the skillet and add in tamari sauce. Stir to coat and let cook for about 3 minutes until everything is coated and most of the tamari sauce has absorbed into the mixture.
Remove from heat and serve over rice, quinoa, etc. Top with sriracha and fresh cilantro if desired. Enjoy!