Preheat the oven to 425°.
Chop the butternut squash into ½ inch cubes.
Coarsely chop the red onion and place it in a blender with the rosemary, Dijon mustard, salt, and olive oil. Blend until smooth
Coat the squash with the blended mixture and place in a baking dish.
Cook the squash for 10 minutes and then stir and cook for another 10 minutes or until it is soft.
In the last 5 minutes of cooking, add the pine nuts.
Remove from the oven and allow to cool.
Add the arugula and balsamic vinegar at the end.