Melt butter in a soup pot over medium heat. Add onions and carrots stirring occasionally, until the onions soften, about 5 to 8 minutes. Do not let the onions or carrots brown. Sprinkle a teaspoon of salt over the carrots and onions as they cook.
Add the stock, water, ginger and strips of orange zest. Cover and simmer until carrots soften, about 20 minutes.
Remove the strips of orange zest and discard.
Working in small batches, pour soup into a blender and purée until completely smooth. Only fill the blender bowl a third full with the hot liquid and keep one hand pressing down on the cap of the blender to keep it from popping off.