01 - Melt butter in a small skillet over medium-low heat. Add sliced onion, granulated sugar, and a pinch of salt. Cook gently, stirring occasionally, until onions are deeply golden and caramelized, about 12 to 15 minutes.
02 - Transfer the caramelized onions and any skillet juices to a blender. Add 2 tablespoons of milk and blend until completely smooth. Strain through a fine-mesh sieve to achieve a silky consistency.
03 - Brew 2 shots of espresso and divide evenly between two cups. Stir in 1 tablespoon of strained onion purée into each cup, mixing thoroughly to combine.
04 - Heat the remaining milk until steaming. Froth using a milk frother or vigorously whisk until creamy and foamy.
05 - Pour frothed milk over the espresso-onion mixture in each cup. Add honey or maple syrup to taste if desired. Stir gently and serve immediately.