01 - In a small skillet over medium-low heat, melt unsalted butter. Add thinly sliced onion, granulated sugar, and a pinch of salt. Cook slowly, stirring occasionally, until onions are deeply caramelized and golden, about 12 to 15 minutes.
02 - Transfer the caramelized onions and pan juices to a blender. Add 2 tablespoons (1 fluid ounce) milk and blend until completely smooth. Strain through a fine-mesh sieve for a silky consistency.
03 - Brew 2 shots (2 fluid ounces) espresso and divide between two cups. Stir in 1 tablespoon of strained onion purée into each cup and blend well.
04 - Heat the remaining milk until steaming. Froth using a milk frother or whisk until creamy and foamy.
05 - Pour frothed milk over the espresso-onion mixture in each serving cup. Add honey or maple syrup for sweetness if desired. Gently stir and serve promptly.