Rinse the lentils then put them into a pot and fill it up with water to soak. Follow instructions on the packaging for soaking.
Heat the oil in a big pot and then add the diced onion, carrot, celery and salt to it.
Saute the onion, carrot and celery on low heat until the onion gets a light brownish caramelized look. It takes around 15 to 20 minutes.
Add black pepper and the garlic then continue frying them on low heat until the garlic releases it's nice garlic scent. This will take around 2 to 3 minutes. Mind not to burn the garlic to dark brown as that would ruin the dish.
Add lentils, tomato, paprika and optional spinach, chorizo. If we use any other vegetables, herbs for flavoring or meat this is the time to put them in. Parsley root, parsnip, basil, oregano, chives, thyme Spanish ham or just bones with a bit of meat on them are all great options to choose from.
Fill the pot up with water. Bring it to boil then simmer the lentils on low heat for about 40 to 60 minutes depending on the type. Check cooking times on packaging.
Roughly 10 minutes before the end of cooking add the chosen vegetables to the soup. Put the lid back on and bring it to boil then lower the heat and simmer the soup for about 5 -10 more minutes or until the vegetables get the desired texture. Take it off heat and let it rest for at least 20 minutes before serving (keep in mind that vegetables are still being cooked by the residual heat).