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