This seemed like a complete impossibility to me, but it works. And it works well.
Start off with a small amount of sugar in a pan - 1 teaspoon or so - on high; watch it carefully. It will liquify then caramelize and fizzle. Right when it's caramelized, throw in 3 onions (cut against the grain into 1/2 rings) and stir them around. Add a few Tablespoons of butter and a pinch of baking soda. Finally, add some water (a 1/2 cup to 1 cup), deglaze, and turn it down. Keep cooking until the water is evaporated, stirring occasionally.