Although the word 'Quesillo' means something different in other Central or Latin American countries (e.g., Mexican string cheese), in Nicaragua, it is exactly this roll. The foundation is a specific soft, stretchy, white cheese, also called 'Quesillo' or 'Cuajada Agridulce' in Nicaragua.
It is prepared incredibly quickly and is the perfect salty, creamy, and sour snack. Traditionally, it is served wrapped in a banana leaf or a small plastic bag to ensure no sauce spills out.
The key element is the distinctly sour onion, which is usually pickled in vinegar. The most authentic flavor is achieved with 'banana vinegar,' but white vinegar works well too.
Slice the onion into thin strips.
Optional (for a milder flavor): Briefly boil the onion (about 2 minutes) in hot water. Drain and cool.
In a bowl, mix the vinegar, water, a pinch of salt, and sugar. Pour over the onion. Let it steep at room temperature for at least 30 minutes, ideally in the refrigerator for 2-3 hours, until the onion softens and absorbs the sourness.
Slightly drain the onion from the brine before serving, but reserve some liquid.
If using Quesillo/Cuajada cheese, it is traditionally served as a long, stretchy roll. You can briefly submerge it in hot water (or heat it in the microwave/pan) to soften and make it stretchable.
Heat the tortillas on a dry skillet or griddle for 1-2 minutes per side until they are hot and pliable (but not crispy).
Prepare a banana leaf or a piece of aluminum foil/bag for serving.
Place a piece of cheese (ideally rolled into a log shape) onto the warm tortilla.
Add a generous spoon of pickled onions and a little of its vinegar brine on top of the cheese.
Top with 1-2 tablespoons of sour cream (Crema). You can add a pinch of salt directly onto the cream, and optionally hot sauce.
Roll the tortilla tightly into a scroll. If using a banana leaf/bag, wrap the roll in it and serve immediately, ideally slightly warm so the cheese is soft.