In a large saucepan over low heat, soften the onions in the butter and cook until caramelized, about 30 minutes. Remove 1/3 of the onions and set aside.
Add the turnip and milk. Bring to a boil over medium heat, stirring frequently and scraping the bottom of the pan so that the milk does not scorch. Reduce the heat, cover and simmer gently until the turnip is tender, about 20 minutes.
Purée the soup in a blender. Strain if desired. Adjust the seasoning.
Ladle the soup into 4 bowls. Spread the reserved caramelized onions on the toast and set a slice on the edge of each bowl.
Can be made a day ahead. Reheat before serving.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.