Make the pickled mustard seeds, if using. Combine the vinegar, water, sugar, and salt in a small pot over medium heat. Bring to a simmer, stir in mustard seeds, reduce the heat, and cook over low heat for 30 minutes or until the mustard seeds are plump. Remove from the heat and let cool to room temperature. Store in the refrigerator for up to 2 weeks.

Make the salad. In a large bowl, combine the vinegar, agave nectar, cucumber, onion, salt, sesame oil, and dill. Toss to coat and chill for 10 minutes. Transfer to a serving bowl, leaving some of the excess water behind. Top with fresh mint, dollops of pickled mustard seeds, if using, and a drizzle of olive oil, if desired. Season to taste and serve.