Place pinto beans in a large stock pot with water to cover; let soak, 8 hours to overnight. Drain.

Cover the beans with about 3 inches water and cook over medium-low heat until tender, 35 to 40 minutes. Drain.

Heat 1/4 cup vegetable oil in a skillet over medium-high heat. Add pumpkin seeds; cook and stir until light brown and popping, about 6 minutes. Let cool slightly, about 5 minutes.

Pour pumpkin seeds and oil into a blender or food processor. Blend until smooth. Add 1/2 the tomatillos, serrano peppers, arugula, spinach, lettuce, parsley, and cilantro into the blender. Puree until smooth. Pour into a large bowl. Repeat with remaining tomatillos, serrano peppers, arugula, spinach, lettuce, parsley, and cilantro.

Heat remaining 1/4 cup oil in a deep pot. Add the tomatillo sauce carefully. Simmer, stirring with a wooden spoon, until thickened, about 30 minutes.

Pour pinto beans, vegetable stock, and hominy into the pot. Bring to a boil. Reduce heat to low. Let simmer, uncovered, until flavors are thoroughly blended, about 15 minutes. Season with salt and pepper.