01 - In a skillet over medium-low heat, melt unsalted butter. Add thinly sliced yellow onion, granulated sugar, and a pinch of salt. Cook gently, stirring occasionally, until onions reach a deep golden caramelization, approximately 12 to 15 minutes.
02 - Transfer the caramelized onion and any skillet juices to a blender. Add 2 tablespoons of milk and blend until completely smooth. Strain the mixture through a fine-mesh sieve for maximum silkiness.
03 - Brew two shots of espresso and pour evenly into two serving cups. Incorporate one tablespoon of strained onion purée into each cup, stirring until well-combined.
04 - Heat the remaining milk until steaming. Froth the milk using a milk frother or whisk until creamy and foamy.
05 - Pour frothed milk over the espresso-onion blend in both cups. Sweeten with honey or maple syrup if desired. Stir gently and serve immediately.