Heat oil in a Dutch oven (or a large saucepan) over medium-high heat.
Once the oil is ready, add the chopped onions and some salt.
Cook until the onions turn a bit soft, then add the coriander.
Stir and cook for a few minutes, then add the lentils.
Keep stirring, then pour six cups of water into the saucepan.
Cover with a lid, then bring to a boil.
Lower the heat and simmer for about 20 minutes.
Add the rest of the ingredients: spinach, potatoes, lemon juice, and more salt (to season).
Mix, then cover with a lid and continue simmering until the potatoes are cooked. This generally takes 25 to 30 minutes for me.
Adjust the seasoning as needed.
Take the soup off the heat and transfer it into bowls.
Serve with slices of lemon, if desired.