Preheat the oven to 250° F.
Mash the garlic with 1/4 teaspoon of the salt in a mortar and pestle, or on a cutting board using the flat side of a knife
Combine the olive oil and the mashed garlic in a medium skillet, and warm over medium heat. When the garlic just begins to turn golden (about 2 minutes), add the 1/2 cup water, cumin, and paprika. Simmer for 1 minute.
Then add the beans and the remaining 1/2 teaspoon salt. Use a masher to mash the beans into the liquid until they are creamy. Simmer, uncovered, stirring constantly to keep the beans from sticking, until all the liquid has been absorbed, 5 to 7 minutes. The beans should be almost dry.
Stir in the lime juice, scallions, and a sprinkling of black pepper. Remove the beans from the heat and set them aside to cool to room temperature.
Combine the masa harina and the warm water in a medium bowl, and knead until the dough is soft and pliable, about 2 minutes.
Divide the dough into 18 portions. Take one portion of dough, leaving the rest in a bowl covered with plastic wrap. Form the dough into a ball, and make a depression in the center. Place the ball in the palm of your hand, and use your other hand to form a cup with walls that are 1/4 inch thick.
Add a spoonful of beans, covering the bottom and leaving 3/4 inch on the sides. Place a T of cheese on top of the beans. Pinch the sides together so that there's an extra little piece at the top, forming a purse. Pinch off the top doughy piece and return it to the bowl. Flatten the filled dough into a pancake about 1/2 inch thick, trying not to let the filling ooze out. Repeat with the rest of the pupusas. Set the pupusas in a single layer on a plate. (You can layer them better between pieces of parchment or wax paper.)
Heat a dry cast-iron griddle over medium-high heat. Fry the pupusas until brown spots appear, 1 to 2 minutes. Flip them over and cook for another minute or so. Keep the pupusas warm in the oven until you finish cooking them all. Serve hot.