Place the olive oil into the bottom of the slow cooker.
Add all of the remaining ingredients, including the stock.
Cover and cook on low for 6-7 hours, or on high for 3-4 hours.
Stir the mixture while it is in the slow cooker. If it is dry, add a little water If it's soup-like, remove lid for a little while, or turn heat to high setting.
If you use red lentils, keep an eye on the time, they will probably cook in 2 hours or so.
Serve with chopped green onions, fresh cilantro, or chopped red onions.
Notes
Store leftovers in the refrigerator. They should stay fresh for about 3 days.