In a large bowl, add the corn masa flour and gradually incorporate the water while kneading until the dough is soft, smooth, and pliable—not overly sticky. Add more water or masa as needed to adjust the consistency.
In a separate bowl, mix the Monterey Jack, mozzarella, and barbacoa until well combined.
Take a portion of masa slightly larger than a golf ball and flatten it into a small disk.
Add about 2 tablespoons of filling into the center, then gently fold the edges over to encase the filling and form a ball.
Flatten the ball back into a thick tortilla shape using your hands, with a little oil or water to prevent sticking.
Cook the pupusas on a hot comal, griddle, or skillet for 3 - 5 minutes on each side, or until golden brown and slightly crisp on the surface.
Repeat the process with the remaining masa and filling.
Serve hot with curtido and mild red salsa.
Enjoy!