01 -  Melt butter in a small skillet over medium-low heat. Add the sliced onion, granulated sugar, and a pinch of salt. Cook gently, stirring periodically, until onions are deeply caramelized and golden brown, 12 to 15 minutes. 
 02 -  Transfer caramelized onions and pan juices into a blender. Add 2 tablespoons milk and blend until completely smooth. Pass the purée through a fine-mesh sieve for a silky consistency. 
 03 -  Brew 2 shots (2 fluid ounces) of espresso and divide evenly between two cups. Stir 1 tablespoon of strained onion purée into each cup until integrated. 
 04 -  Heat remaining milk until steaming. Froth using a milk frother or whisk until creamy and foamy. 
 05 -  Pour frothed milk over the espresso-onion mixture. Sweeten with honey or maple syrup as desired. Stir gently and serve immediately.