Pluck the spinach leaves and discard the stems, wash the spinach leaves in several changes of water. Drain and set aside. Wash the dal and drain.
Pressure cook the spinach and dal with 1 & 1/2 cups of water for 10 minutes after the cooker reaches full pressure.
Cook & blend the spinach & dal to smooth puree. Melt butter in a pan over moderate heat, add flour & cook.
Stir continuously till it sizzles, pour in the milk & stir vigorously to avoid any lumps. Cook, stirring continuously till it comes to a boil.
Add the pureed spinach and dal, salt and pepper. Simmer for 4-5 minutes, stirring occasionally. Serve hot.