Cook the spinach in pot of boiling water for 1 minute.
Drain, rinse under cold water, drain again, and squeeze dry.
Place the spinach, buttermilk, half of the onion, chiles, and the lime juice in food processor and pulse until the spinach is finely chopped and the mixture is well combined. Do not overblend.
Mash the avocado in a bowl.
Stir the spinach mixture with the tomatoes, cilantro, and remaining onion.