In the pre-heated pressure cooker add the olive oil, celery, onion, carrot, and sauté until the onion begins to soften (about 5 minutes). Add potatoes, stock, bay leaf, and lentils and mix well.
Close the lid and set the valve to the pressure position. Cook for 10 minutes at HIGH pressure.
When time is up, open the pressure cooker with the natural release method.
When the pressure is released, open the lid and remove the bay leaf. Mix in the salt and spinach leaves until they are wilted. Squirt with lemon before serving.