Make the Pickled Red Onions

1. In a small saucepan, combine apple cider vinegar, water, sugar, and salt. Heat over medium heat, stirring until the sugar and salt dissolve.
2. Remove from heat and pour the mixture over the thinly sliced red onions in a heatproof jar or bowl.
3. Let sit for at least 30 minutes at room temperature or refrigerate for up to a week.

Prepare the Shrimp Filling

1. Heat 1 tablespoon of vegetable oil in a 12-inch nonstick skillet over medium-high heat. Add 6 sliced garlic cloves and cook, stirring, for 30 seconds until fragrant.
2. Add 1 pound of shrimp to the skillet. Season generously with salt and pepper. Cook for about 2-3 minutes, flipping once, until just shy of opaque in the center. Remove shrimp and set aside.
3. In the same skillet, add 10 ounces of baby spinach and a pinch of salt. Cook for 1-2 minutes until wilted. Remove from heat.

Assemble the Quesadillas

1. Lay out 4 tortillas on a flat surface. Sprinkle half of the 12 ounces of shredded Mexican blend cheese evenly across the tortillas, leaving a small border around the edges.
2. Divide the black beans and shrimp-spinach mixture evenly across the tortillas, then top with the remaining shredded cheese.
3. Place the remaining 4 tortillas on top to form the quesadillas.

Cook the Quesadillas

1. Wipe out the skillet and return it to medium heat. One at a time, cook each quesadilla for 2-3 minutes per side, or until the tortillas are golden brown and the cheese has fully melted.
2. Transfer to a cutting board and repeat with the remaining quesadillas.

Serve

1. Slice the quesadillas into wedges.
2. Stir 1 tablespoon of water into the 1/4 cup sour cream to thin it slightly, and drizzle over the quesadilla wedges.
3. Top with pickled red onions for added flavor and crunch.