Bring a pot of water to a boil; add the cashews and boil for 20 minutes. Drain and rinse with cold water to cool them off.
While the cashews are boiling, add miso paste, lemon juice, vinegar, and 3 1/2 tablespoons water to a blender cup and set aside.
Combine mayonnaise, parsley, dill, mushroom powder, garlic powder, onion powder, and pepper in a mixing bowl with a whisk; it will be very thick. Transfer to the refrigerator until cashews are done boiling.
Once the cashews are boiled and rinsed, add them to the blender cup and blend until very smooth.
Combine the blended cashew mixture into the mayonnaise mixture and whisk to combine.
Vegan Cashew Ranch Dip · Dinner Match Lab | Dinner Match Lab