In a heavy pan with a lid, melt butter. Add onions and salt; cover. Cook the onions on low heat for 40 to 45 minutes, stirring regularly. The onions should not be overly brown and will become soft and sweet.
Prepare an ice bath for the dandelion greens. Fill a medium-sized bowl with water and ice. In a saucepan, boil water and add the dandelion greens. Cook for 1 to 2 minutes, until dark green. Take the greens out of the water and place them in the ice water. Drain the greens with a colander, squeeze out the excess water and set them aside.
Remove the onions from the pan. Add the heavy cream and reduce by a third over medium heat. Add the onions and dandelion greens to the cream. Continue to simmer for 3 to 5 minutes. Add the bacon, curry powder, a few drops of lemon juice and more salt, if desired.
* Dandelions greens can be foraged or found your farmers market or local grocery. If harvesting wild greens, make sure to avoid areas sprayed with herbicides or pesticides. Wild greens are most tender early in the spring.