To create this widget you need to select Custom widget (last widget in dropdown add widget menu)
Segment: Organic Traffic
Dimension : Month of the Year (for example)
Metric: goal Completions All
You can choose to track all goals completion.
Or you can choose to track organic traffic for specific goals:
During the last step you can rename Custom widget “Specific Goals Organic traffic”, choose widget style and size, and you can also select specific data you wish to hide from widget.
In this case the widget will by default display both goal completion number and change on monthly bases.