01 - In a small skillet over medium-low heat, melt the butter. Add the sliced onion, sugar, and a pinch of salt. Cook slowly, stirring occasionally, until the onions are deeply caramelized and golden, approximately 12 to 15 minutes.
02 - Transfer the caramelized onions and any juices from the skillet to a blender. Add 2 tablespoons of milk, blending until completely smooth. Strain the mixture through a fine-mesh sieve to achieve a silky texture.
03 - Brew 2 shots of espresso and divide evenly between two cups. Add 1 tablespoon of the strained onion purée to each, stirring thoroughly.
04 - Heat the remaining milk until steaming but not boiling. Froth using a milk frother or vigorously whisk until creamy and aerated.
05 - Pour the frothed milk over the espresso-onion blend in each cup. Sweeten with honey or maple syrup if desired. Stir gently and serve immediately.