01 - In a small skillet over medium-low heat, melt the butter. Add the sliced onion, granulated sugar, and a pinch of salt. Stir occasionally and cook until the onions are deeply golden and caramelized, about 12 to 15 minutes.
02 - Transfer the caramelized onions with pan juices to a blender. Add 2 tablespoons of milk and process until fully smooth. Pass the mixture through a fine-mesh sieve to achieve a silky purée.
03 - Brew 2 shots (2 fluid ounces) of espresso and divide between two serving cups. Add 1 tablespoon of onion purée to each cup, stirring thoroughly to integrate.
04 - Heat the remaining milk until steaming but not boiling. Froth using a milk frother or whisk until creamy and foamy.
05 - Pour frothed milk over the espresso-onion mixture in both cups. Optionally sweeten with honey or maple syrup. Stir gently and serve immediately while hot.