In a medium saucepan, heat the olive oil over medium heat. Add the onion and sauté until translucent, 3 to 4 minutes. Add the beans, reserved can liquid, kosher salt, chili powder, and black pepper. Turn the heat to medium low and cook for 10 to 15 minutes, stirring often and smashing the beans toward the end of the cook time, until most of the liquid has cooked out and the texture is mashed and thick. Scrape the bottom and sides of the pan as necessary. Taste and season with additional salt as desired.