Fry garam masala, ginger and garlic and chilli in oil for a minute, then add the onions and lentils stirring until coated and onions are translucent, approx 5 mins.
Stir the cubes of potato in until coated, then add the spinach.
Reduce heat, and simmer until the potato has cooked through, testing with a knife after 10 mins and then every 5 mins ish.
When the curry gets too dry, keep adding some more water, you will need a fair bit, as the lentils are thirsty! Taste and as salt as desired.