Step 1: Prepare the Oven and Filling
First things first, preheat your oven to a hot 425°F (220°C). The high heat is crucial for melting the cheese quickly and crisping up the tortilla. While the oven preheats, prepare your delicious filling. In a large bowl, combine the 4 cups of grated cheese, the 2 well-drained cans of diced green chilies, and the ½ cup of finely diced onion. Use a fork or your hands to toss everything together until the chilies and onion are evenly distributed throughout the cheese.

Step 2: Prepare the Baking Sheet
This step is the secret to the ultimate crispy texture. Take a large, rimmed baking sheet. Using a pastry brush, generously coat the entire surface of the pan with the 4 tablespoons of melted butter. Don’t be shy here; you want a nice, even layer. Once buttered, sprinkle the pan generously with coarse Kosher salt. This seasons the outside of the quesadilla and adds a wonderful crunch.

Step 3: Assemble the Quesadillas
Arrange the small “street taco” tortillas on the prepared baking sheet in a single layer. Depending on the size of your pan, you should be able to fit about 7 to 8 tortillas without them overlapping. Overlapping will cause them to steam rather than crisp, so give them a little space.

Step 4: Add the Filling
Take a heaping tablespoon of the cheese mixture and place it on one half of each tortilla. Use the back of the spoon or your fingers to spread the mixture evenly over the entire surface of the tortilla, leaving a very small (about ⅛-inch) border around the edge. An even layer ensures every bite has the perfect ratio of cheese to tortilla.

Step 5: The First Bake
Place the baking sheet in the preheated 425°F oven. Bake for approximately 5 minutes. You’re looking for the cheese to be completely melted, bubbly, and gooey. The edges of the tortillas should just be starting to turn a light golden brown.

Step 6: Fold and Finish
Carefully remove the hot baking sheet from the oven. Working quickly while the cheese is still molten and acting as a glue, use a spatula or tongs to fold each tortilla in half, creating a classic half-moon shape. Gently press the edges together to seal them. Remove the folded quesadillas from the hot pan and place them on a wire cooling rack. This is important: cooling them on a rack allows air to circulate, preventing the bottoms from getting soggy.

Step 7: Repeat and Store or Serve
Repeat the process with the remaining tortillas and filling, re-buttering and salting the pan for each batch. If you plan to serve them later, allow the quesadillas to cool completely on the wire rack before storing them in an airtight container in the refrigerator for up to 2 days.

Step 8: Reheating for Serving
When you’re ready to serve, take the quesadillas out of the refrigerator and let them sit at room temperature for about 15-20 minutes. Preheat your oven to 350°F (175°C). Arrange the quesadillas in a single layer on a clean baking sheet (no need to butter the pan this time). Heat for about 5-7 minutes, or until they are heated through and the exterior has re-crisped.

Step 9: Slice and Serve
Once heated, remove them from the oven. For the perfect appetizer presentation, use a sharp knife or a pair of kitchen scissors to slice each half-moon quesadilla in half again, creating two neat wedges. Arrange them on a platter and serve immediately while they are warm and crispy.