01 -  In a small skillet over medium-low heat, melt the unsalted butter. Add the sliced onion, granulated sugar, and a pinch of salt. Cook gently, stirring occasionally, until the onion is deeply caramelized and golden brown, approximately 12 to 15 minutes. 
 02 -  Transfer the caramelized onion and any remaining juices to a blender. Add 2 tablespoons of milk and blend until very smooth. Pass the mixture through a fine-mesh sieve for an ultra-silky texture. 
 03 -  Brew 2 shots of espresso and divide evenly into two serving cups. Stir 1 tablespoon of the strained onion purée into each cup until well incorporated. 
 04 -  Heat the remaining milk in a saucepan until steaming but not boiling. Froth the milk using a milk frother or whisk to achieve a smooth, creamy foam. 
 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.