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