Heat oil over medium-high heat, add onion, and sauté, stirring frequently, until soft and golden – about 10 minutes.
Meanwhile, wash and zest one of the lemons into one bowl, then juice both of them into the other.
Add lemon zest, water, and lentils to the onions. Stir well, cover, and bring to a boil. Reduce heat and simmer about 30 minutes, until lentils turn to mush.
Remove from heat and stir in remaining ingredients.
Cover and let sit until spinach wilts.
Garnish and serve hot.