Grilled Spring Onions with Pistachio Butter

Name: Grilled Spring Onions with Pistachio Butter
Ingredients: 1 cup rice vinegar; 1/4 cup sugar; 1 bay leaf; Kosher salt; 1 cardamom pod; 1 teaspoon black peppercorns; 1 teaspoon coriander seeds; 1 1/2 cups whole pitted prunes; 1 1/4 cups unsalted pistachios; 1 garlic clove, finely grated; 1 teaspoon grated lemon zest; 6 tablespoons extra-virgin olive oil, plus more for drizzling; 25 young spring onions or large scallions, trimmed; 1 cup Greek yogurt; 1/2 teaspoon Urfa or Aleppo pepper (see Note); Mint leaves, for garnish