Preheat oven to 425 degrees. Rub pork tenderloin with 2 tablespoon olive and sprinkle with taco seasoning and gently rub in. Heat a cast iron skillet over medium-high heat, add 1 tablespoon olive oil. Once pan is hot add sliced onion and sauté until light, translucent, about 3 minutes; remove from pan and return pan to heat. Add pork to pan; cook 4 minutes or until lightly browned, turn pork, place sautéed onions over pork and place pan in oven. Bake for 10-15 minutes or until thermometer reads 145 degrees. Remove from oven, tent with foil and allow to rest for 10 minutes. Slice and serve on tostadas topped with salsa.