Make the dressing by combining the dressing ingredients in a blender or blending with an immersion blender. This can be made up to 5 days ahead and stored in the refrigerator until time for use.
Caramelize the onion by heating a cast-iron skillet over medium heat and melting one tablespoon of butter in the skillet. Arrange the onion slices in a single layer in the skillet. Turn the heat down to low and cook, stirring occasionally, until the onion has browned and caramelized (10 to 15 minutes).
Cook the farro according to the package directions. When the farro is finished, drain off any extra water, pour the farro into a large salad bowl, and set it aside to cool slightly.
Peel and segment the grapefruit and oranges, then chop them into small bite-sized pieces.
Add the citrus pieces and caramelized onion to the farro. Toss with rosemary.
Warm the dressing in a small saucepan, then pour it over the salad to taste. Toss and serve warm.