Grill sweet corn over open flame until lightly charred. Let cool and cut kernels off cob.
In a large pot, melt 7 tablespoons butter. Add onions and cook for about 10 minutes, stirring frequently. Add sweet corn, reserving ¼ cup for garnish. Add water and bring to a boil. Reduce heat and cook for 5 minutes.
Remove from heat and puree with an immersion blender or food processor. Let cool.