Add the 1.5 cups of rice vinegar, 1/4 cup of agave nectar, 1/4 tsp of sea salt, 2 tbsp of pepper corns, and 1/4 cup of water into a cooking pot. Bring to a boil, then simmer for 2 minutes. Then, pour the mixture directly into the mason jar and cap immediately. Add extra water if needed to submerge the onions completely.