Make the brine. Heat a medium saucepan over medium-high heat. Add water, vinegar, salt, sugar, garlic, and mustard seed to the pan. Cook until it begins to simmer and the sugar dissolves. Remove from heat.
1 ¼ cups water, 1 ¼ cups white vinegar, 2 Tablespoons Kosher salt, 1 ½ Tablespoons sugar, 4 cloves garlic, 2 teaspoons whole mustard seeds