Saute the onion and garlic in the oil with black pepper. When the onion is clear and becoming slightly golden, add the lentils with enough water to cover plus one inch. Bring to a boil and then turn down heat to simmer. Check the lentils after 30 minutes to ensure that there is sufficient water. After 40 minutes (or 45 if the spinach is thawed), add the spinach. Ten minutes later stir the spinach thoroughly and then cook 10 minutes more for a total of 60 or 65 minutes.
Serve over brown rice. It may need a bit of salt.