In a bowl, combine all salsa ingredients; set aside.
Cut pork into 1-inch-thick medallions. Cut medallions into ½-inch-thick strips. In a bowl, toss pork with chile powder and ½ tsp salt; set aside.
Heat a large skillet on medium-high. Add tortillas in batches and heat until warm and pliable, about 1 minute per side. Stack tortillas, wrap in foil and set aside.
Reduce heat to medium and heat oil in skillet. Add sliced onions and sauté until beginning to soften, 3 minutes. Add pork and continue to sauté until no longer pink in center, about 4 minutes. Transfer to a plate and cover with foil. Add chard leaves to pan and sauté, tossing with tongs, until chard has wilted, about 2 minutes. Divide pork, chard and salsa among tortillas.
Red beet and chile pepper salsa makes for a striking and flavorful topper on these oregano and sriracha–marinated steak tacos. Because this salsa requires a little baking time to cook the beet, you can make it entirely up to 1 day in advance and refrigerate until serving.