Lay the large flour tortilla flat on a clean work surface. Distribute half of the shredded cheddar and half of the Monterey Jack evenly over one half of the tortilla, leaving a 1/2-inch border.

Top the cheese layer with your chosen cooked protein (about 3/4 cup). If using onions or jalapeño, sprinkle them evenly over the protein. Add the remaining cheese on top of the protein to help the quesadilla seal when melted.

Fold the empty half of the tortilla over the filled half, creating a semicircle. Press gently to help the layers adhere.

Cooking

Heat a nonstick skillet or cast-iron pan over medium heat. Add the butter and swirl to coat the pan so the quesadilla will get golden and crisp.

Place the folded quesadilla in the pan. Cook 2 to 3 minutes until the bottom is golden brown and the cheese begins to melt. Carefully flip using a spatula and cook another 1.5 to 3 minutes until the second side is golden and the cheese is fully melted. If the tortilla is browning too quickly before the cheese melts, reduce heat to medium-low and cover the pan briefly (30–60 seconds) to speed melting without burning.

Remove the quesadilla to a cutting board and let rest 1 minute to let cheeses set slightly, which makes slicing easier.

Slice the quesadilla into 3–4 wedges with a sharp knife or pizza cutter.

To serve

Transfer wedges to a plate and garnish with chopped cilantro. Serve with salsa and sour cream on the side for dipping.

Optional: squeeze a little lime over the quesadilla or add hot sauce to taste.