Add the cashews to a bowl and cover with water. Let the cashews soak at room temperature for 2 hours.
About 45 minutes before the cashews are finished soaking, combine the chopped onions, olive oil and a pinch of salt in a medium skillet over medium-low heat. Cook the onions, stirring occasionally, until they are deep golden brown and caramelized, about 30 minutes. To prevent the onions from burning and sticking to the bottom of the skillet, you can add a splash of water, 1 tablespoon at a time, as needed. Let caramelized onions cool slightly.
Drain the cashews and add them to the bowl of a food processor or high-speed blender with the beans and 1/2 cup water. Blend the mixture until smooth, about 5 minutes, stopping the mixer and scraping down the bowl occasionally.
Add the lemon juice, vinegar, onion powder, paprika and 1/2 teaspoon salt, and blend again until smooth. Transfer the mixture to a bowl. Reserve about 1 tablespoon onions for garnish, then add the rest to the bowl. Fold the onions into the mixture, cover and refrigerate for at least 1 hour and up to overnight. (Overnight is ideal for the best flavor.)
Just before serving, give the dip a good stir and adjust the seasoning with lemon juice and salt as desired. Top with the reserved onions and chives, if desired, and a few grinds of black pepper. Serve with potato chips or crudités. Store leftovers covered in the fridge for up to 3 days.
