In a saucepan over medium heat, melt the butter. Add the onions. Season with salt. Saute for two minutes. Add the mango. Continue to saute for 2 minutes. Add the vinegar and the sugar. Bring the mixture to a boil. Whisk the cornstarch with the water. Whisk until smooth. Stir the slurry into the mango mixture. Bring the mixture back to a boil. Continue to cook for 4 minutes, stirring constantly. Remove from the heat and cool completely. Store in an airtight container until ready to use.