01 - In a small skillet over medium-low heat, melt the butter. Add the sliced onion, granulated sugar, and a pinch of salt. Cook gently, stirring occasionally, until the onions are deeply caramelized and golden, about 12 to 15 minutes.
02 - Transfer the caramelized onions and any pan juices to a blender. Add 2 tablespoons milk and blend until completely smooth. Strain through a fine-mesh sieve for a silky texture.
03 - Brew 2 shots of espresso and pour evenly into two cups. Add 1 tablespoon strained onion purée to each; stir thoroughly to combine.
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. Gently stir and serve immediately.