In a large pot, heat the onions with butter or olive oil and cook until softened.
Add the corn and cook over medium-high heat for 1-2 minutes.
Add the vegetable broth and milk.
Transfer to a blender, add the sugar and a half-teaspoon of salt (or so), then blend until pureed.
Return the soup to your pot and heat just before the soup reaches boiling.
If you want the corn to be less like chowder and more like soup, pour in an additional 1/2-1 cup of milk or broth to thin it out (I like it thin, so always add that extra bit of liquid.) Serve immediately topped with crab salsa.