In a large mixing bowl, add the masa, water, and salt and stir together.
Using your hands, start kneading the ingredients together until a soft, smooth, moist dough forms. You may need to add a little more water, up to ¼ cup to get it to the right consistency. You know it is ready when you can gently pinch some of the dough together and it doesn’t crack.
Now, use a dish towel to cover the bowl and let the dough rest for 15 minutes.
Toss together the mozzarella and queso fresco in a small bowl and set aside.
Divide the dough into 8 equal portions, you can do this by weighing them on a kitchen scale or just eyeballing it.
Grab one of the dough pieces and pat it into a large circular disc in your hands.
Then add 1/8th of the cheese mixture to the center of the dough disc and fold it and pinch the dough closed over the cheese. Roll it into a ball.
Gently pat and press the ball into a round disc about 1/4th inch thick. Repeat with the remaining dough and cheese to make all 8 dough pockets.
Heat a griddle over medium heat. Once hot, spray with cooking spray. Cook for about 5-6 minutes per side until they're golden brown and slightly charred. Place them on a wire rack to cool down while you are cooking the remaining pupusas. Serve and enjoy immediately.