Place a medium saucepan on the stove and add apple cider vinegar, water, sugar, and salt. Set the heat to medium and stir continuously until the sugar and salt are fully dissolved. Ensure the mixture remains below boiling to avoid any unwanted bitterness. This process should take about 5 minutes.

Add black peppercorns, mustard seeds, and sliced garlic to the saucepan over medium heat. Stir gently to combine, ensuring the spices are evenly distributed. Allow them to simmer for about 5 minutes, releasing their aromatic qualities into the liquid. You should notice a fragrant aroma as the spices infuse.

Begin by placing the thinly sliced red onions into a clean, heatproof jar. Ensure the jar is thoroughly sanitized to avoid any potential spoilage. Arrange the onion slices in even layers, pressing them down gently to maximize space and ensure uniform pickling throughout the jar.

Carefully pour the hot vinegar mixture over the sliced onions in the jar, ensuring they are completely covered. The heat from the liquid will allow the onions to soften and absorb the tangy flavors more effectively. Let them sit for at least 30 minutes to develop the desired taste.

If you prefer a spicier kick, sprinkle red pepper flakes into the jar. Secure the lid tightly, then allow the jar to sit at room temperature for about 30 minutes. This cooling period helps the flavors meld. Once cooled, transfer the jar to the refrigerator to chill thoroughly.

Place the pickled onions in the refrigerator for a minimum of 30 minutes to allow the flavors to blend thoroughly. During this time, the onions will develop their characteristic pink color, indicating they are ready to enhance your dish with their tangy essence.

After 30 minutes, sample the onions to assess their tanginess. If they lack the desired sharpness, allow them to continue marinating in the refrigerator for an additional 15 to 30 minutes. This extended time will enhance their flavor profile, providing a more pronounced pickled taste.

To achieve a sweeter taste, increase the sugar in the pickling liquid. Stir thoroughly over medium heat until the sugar is fully dissolved, ensuring no granules remain. Once dissolved, remove from heat and pour the mixture evenly over the sliced onions, covering them completely.

If the onions taste overly pungent, rinse them under cold water for about 10 seconds before serving. This quick rinse can help reduce the sharpness while maintaining the pickled character. Ensure the water is cold to avoid altering the texture or flavor of the onions.

Transfer any leftover pickled onions to a clean, airtight container, ensuring they are fully submerged in the pickling liquid. Place the container in the refrigerator promptly to maintain their crispness and tang. Consume within two weeks for optimal taste and texture. Always use a clean utensil to remove onions, preventing contamination.