Quick Pickled Cucumber and Onion: Mix brown rice vinegar and sugar together. Let sit until sugar dissolves. Add thinly sliced cucumber, onion, and serrano pepper. Let marinate at least an hour to overnight.
Teriyaki Tofu: Mix tamari, mirin, brown sugar, garlic and ginger in a bowl. Slice drained and pressed tofu into ½" squares. Put tofu into marinade after sugar dissolves. If you are in a hurry, heat marinade on stovetop or microwave until sugar dissolves. Let sit for at least an hour to overnight. Add sesame oil to a hot pan. Add tofu and cook on medium high heat until tofu is slightly crispy on both sides. Add teriyaki marinade and cook until it is almost totally dissolved.
Tacos: Heat corn tortillas on stovetop or in microwave until warm and pliable. Pile tofu on torillas. Add pickled cucumber and onion. Add chopped cilantro. Drizzle sriracha sauce on top.