01 - In a small skillet over medium-low heat, melt unsalted butter. Add sliced onion, granulated sugar, and a pinch of salt. Cook gently, stirring occasionally, until onions are caramelized to a deep golden color, approximately 12 to 15 minutes. Avoid high heat to develop optimal sweetness and depth.
02 - Transfer the caramelized onions and pan juices to a blender. Add 2 tablespoons milk and blend thoroughly until the mixture is smooth. Strain through a fine-mesh sieve to ensure a silky consistency.
03 - Brew two shots of espresso and distribute evenly into two serving cups. Incorporate 1 tablespoon strained onion purée into each cup, stirring until fully integrated.
04 - Heat the remaining milk until steamy. Froth using a milk frother or vigorously whisk until luxuriously creamy and foamy.
05 - Layer frothed milk over the espresso-onion blend in each cup. Adjust sweetness with honey or maple syrup to taste. Stir gently and serve immediately for peak flavor.