01 - In a small skillet set over medium-low heat, melt the butter. Add the sliced onion, sugar, and a pinch of salt. Cook gently, stirring occasionally, until the onion becomes deeply caramelized and golden, about 12–15 minutes. Slow cooking enhances the flavors.
02 - Transfer the caramelized onions and any juices into a blender. Add 2 tablespoons of milk and blend until the mixture is completely smooth. Press the purée through a fine-mesh sieve for a silky finish.
03 - Brew 2 shots of espresso and distribute evenly between two cups. Stir in 1 tablespoon of strained onion purée into each cup, mixing thoroughly.
04 - Steam the remaining milk until hot. Using a milk frother or whisk, aerate until creamy and foamy.
05 - Pour the frothed milk over the espresso-onion mixture in each cup. Optionally, sweeten with honey or maple syrup. Stir gently and serve at once.