1. Make the Pickled Red Onions:

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

2. Prepare the Shrimp Filling:

• Heat the 1 tablespoon of vegetable oil in a 12-inch nonstick skillet over medium-high heat. Add the 6 sliced garlic cloves and cook, stirring, for 30 seconds until fragrant.
• Add the 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.
• In the same skillet, add the 10 ounces of baby spinach and a pinch of salt. Cook for 1-2 minutes until wilted. Remove from heat.

3. Assemble the Quesadillas:

• 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.
• Divide the black beans and shrimp-spinach mixture evenly across the tortillas, then top with the remaining shredded cheese.
• Place the remaining 4 tortillas on top to form the quesadillas.

4. Cook the Quesadillas:

• 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.
• Transfer to a cutting board and repeat with the remaining quesadillas.

5. Serve:

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