In a medium-sized bowl, combine ½ cup of sliced red onion, and 1 orange peeled and diced. Add the red wine vinegar, 2 tablespoons of olive oil, pinch of sugar, and a sprinkle of salt and pepper. Toss gently to coat and set aside to marinate.
Heat remaining 1 tablespoons of olive oil in a large skillet over medium-high heat. Add remaining red onion and cook until softened, around 4 minutes. Add the pork and mojo spice mix and continue cooking, breaking up the meat as it cooks, until no longer pink, around 5 minutes more.
In a small bowl, combine the juice from the remaining orange, the juice from 2 limes, and the soy sauce. Add to the skillet with the pork, then add the black beans and gently combine, cooking until the liquid is reduced by half, around 5 minutes. Stir in ⅔ of the queso fresco until melted.
To serve, divide mojo pork between tostadas, then scoop orange salsa from bowl, letting the liquid remain in the bowl and top each tostada. Sprinkle with remaining queso fresco and cilantro. Serve immediately.