Whole Wheat–Oat Waffles

Name: Whole Wheat–Oat Waffles
Ingredients: 1 lemon, zest removed in wide strips, halved; ⅓ cup granulated sugar; 1 (10–12-oz.) bag frozen blueberries; 1 Tbsp. cornstarch; 1 cup (125 g) whole wheat flour; ½ cup plus 2 Tbsp. (60 g) quick-cooking old-fashioned oats; 1 tsp. baking powder; ½ tsp. baking soda; 1 tsp. Diamond crystal or ½ tsp. Morton kosher salt; 1½ cups plain yogurt (not Greek); 4 Tbsp. unsalted butter, melted, plus more room-temperature for serving; 1 large egg; Nonstick vegetable oil spray or melted unsalted butter (for iron)