Begin by adding cashews, vinegar, lemon juice, miso (if using), salt, and hot water to a high-speed blender or food processor. Blend until smooth, adding more water as needed to help achieve a smooth consistency. Set aside.

Preheat the oven to 400 degrees F and line a baking sheet with a mat or parchment. Slice rhubarb into 2-inch pieces and toss with oil. Add to the baking sheet and sprinkle with salt. Roast until soft, about 15 minutes. Set aside once done.

While rhubarb is roasting, prepare caramelized onions by adding oil, onion, salt, brown sugar, and balsamic (if using) to a cast iron pan over medium-low heat on the stovetop. Stir to combine and cook until fragrant and browned, about 15 minutes. Stir to prevent burning. Set aside. 

To prepare sandwich, toast bread to your liking and butter each side. On one side of the sandwich, layer rhubarb and 2 teaspoon honey or agave. On the other slice, add 2 TBS cashew cheese, 1-2 TBS of caramelized onions, a few leaves of mint, and ½ cup of sprouts. Sandwich together and enjoy immediately.