In the container of a food processor or blender, combine half of the spinach with the buttermilk. Puree until smooth. Transfer to a saucepan, and stir in the rest of the spinach. Stir together the cornstarch and water; mix into the saucepan along with the sugar, peanuts (see Cook's Note), and salt. Set aside.

Heat olive oil in a small skillet over medium heat. Add cumin seeds, and when they begin to sizzle, stir in the serrano peppers. (You can slice the peppers in two for added flavor and spiciness--or use fewer chiles, if desired.) When the peppers begin to soften slightly, scrape the contents of the pepper pan into the buttermilk mixture.

Bring the buttermilk mixture to a boil, stirring constantly. Remove from heat immediately, and serve hot.