Heat the oil in a skillet set over medium-high. Add the prawns and cook 1 minute per side. Transfer prawns to a plate.
Add the onions and garlic to the skillet and cook until softened, about 2 minutes. Mix in the cumin, turmeric and cardamom and cook 1 minute more. Add the spinach and stock to the skillet, bring to a simmer, and simmer 1 minute, just until the spinach wilts. Transfer the spinach mixture to a food processor or blender. Pulse until smooth.
Return spinach mixture to the skillet. Add the prawns and simmer and cook 1 minute more, or until the prawns are cooked through. Season saag with a bit of salt, if needed, and serve.
Note: After peeling, if you wish to devein the prawns, use a small paring knife to make a lengthwise slit along the back of each prawn. Pull out, or rinse out with cold water, any dark vein material. Pat prawns dry before using.