Drain and rinse the cashews and transfer them into a high-speed blender. Add lemon juice and salt along with 1/2 cup water. Blend until smooth and creamy.
Heat the olive oil in a heavy-bottomed skillet over medium heat. Add the onion and cook until the onion becomes translucent, around 1 minute. Add the squash, rosemary, and salt and stir well.
Partially cover the skillet and cook until the squash has softened a bit, about 5 to 7 minutes. Add a few tablespoons of water if the pan begins to look dry.
Add the white wine and deglaze the pan, loosening any small, delicious bits that may be stuck to the bottom of the pan.
When the wine has been absorbed, remove the pan from the heat, cover and let it sit for 5 minutes. The butternut squash is ready when it is soft and beginning to break apart.
In a large bowl, combine the cashew cream, butternut squash mixture, and radicchio. Blend half of the mixture, mix that into the remaining mixture.
Preheat the oven to 400°. Transfer the mixture to a small baking dish (such as 9-inch x 9-inch square or a 6-inch round) and bake for 20 to 30 minutes, until the dip has puffed up slightly and has a golden brown surface.
Let the dip sit for 5 minutes before serving. It’s best served warm but can also be served at room temperature with a selection of crostini, crackers, or vegetable crudites.