Prepare the pickled onions: Thinly slice 1 large red onion and place in a bowl. Add 1/2 cup white vinegar, 1/4 cup water, 1 teaspoon salt, and 1 teaspoon sugar. Mix well and let sit for at least 15 minutes while preparing other components.

Make the corn tortilla dough: In a large mixing bowl, combine 2 cups masa flour (corn flour) with 1 1/4 cups warm water and 1/2 teaspoon salt. Knead the dough for 3-4 minutes until smooth and pliable. The dough should be moist but not sticky. Cover with a damp cloth.

Form the tortillas: Divide dough into 8 equal portions and roll into balls. Place each ball between two pieces of plastic wrap or wax paper. Using a tortilla press or heavy flat-bottomed pan, press each ball into a 6-inch round tortilla.

Cook the tortillas: Heat a large cast iron skillet or griddle over medium-high heat. Cook each tortilla for 45-60 seconds per side until lightly charred spots appear. Keep warm wrapped in a clean kitchen towel.

Prepare the cheese: Cut the mozzarella cheese into 8 long strips, about 1/2 inch thick and 4 inches long. If using queso fresco, crumble it into chunks.

Assemble the quesillo: Place a warm tortilla on a plate. Add a strip of cheese down the center. Fold the tortilla over the cheese like a soft taco.

Add toppings: Drizzle 2 tablespoons of sour cream over each quesillo. Top with a generous portion of pickled onions (drain excess liquid first).

Serve immediately: The quesillo is best eaten while the tortillas are still warm and the cheese is soft. Traditionally, these are wrapped in plastic for portable eating, but serving on plates works perfectly for breakfast at home.