Melt the butter in a large saute pan over medium-high heat. Add the onions, lemon zest, ginger, and 1/2 teaspoon salt and cook until the onions start to turn translucent, about 5 minutes
Add the coriander, 1/4 cup of the brown sugar and half the lemon juice and turn the heat to low. Cook, stirring frequently, until the onions start to cook down and the liquid is evaporated, about 15 minutes. Add 1/3 cup water every 10 minutes and continue to cook until the onions are thick and golden brown, about 55 minutes. Stir in the remaining 1/4 cup brown sugar and the lemon juice and cook until the brown sugar melts, 3 to 5 minutes more. Season to taste.