Place beans and avocado in a large mixing bowl and mash well with a fork until thick and chunky yet creamy (similar to a chunky guacamole or hummus).
Fold in onion, carrot, and spinach.
Whisk together miso paste, cider vinegar, and citrus juices and zest in a separate bowl.
Fold into bean mixture with nutritional yeast, hemp or sunflower seeds, and cayenne, if using, until well combined and creamy.
Spread on English muffins, and season to taste with salt, pepper, harissa, and chives.
(Mixture can also be chilled 15 to 30 minutes before serving.)