Make these at least a day in advance for best results. The onions keep 3-4 weeks in the fridge.
Place onions and peppercorns in a sealable jar or container. Mason jars work the best.
On the stove, bring the vinegar, sugar, and water to a boil. Stir to dissolve the sugar and set aside. Allow the mixture to cool to just above room temperature. Pour over the onions. Seal and store in the fridge.
For Ceviche:
Chop the raw fresh shrimp into small pieces and place in a bowl. Juice the limes over the shrimp. Toss the coat. Make sure you are mostly covering the shrimp because it needs the acid from the lime to “cook”. Put the shrimp in the lime juice in the fridge for 2-3 hours.
Just before the shrimp is done “cooking” in the lime juice. Chop the remaining ingredients.
Stir the avocado, tomato, jalapeno, onions, and cilantro into the shrimp. Season with salt and pepper.