Heat the olive oil over medium heat in a large, heavy bottomed pot or crockpot (if it has a saute setting). Add the chopped celery, carrot and onion and cook, stirring often until tender and the onions are transparent, about 10 minutes. Turn the heat down to low, if need be.
Add the peeled and smashed garlic and cook for another 2 minutes, being careful not to burn the garlic.
Add the canned tomatoes. Tie the rosemary and the bay leaf into a small bundle with a piece of kitchen string (this helps the rosemary from falling apart into the soup) and add. Cook for another 5 minutes over medium-low heat.
Add the lentils, 8 cups of water or broth, salt (if you are using canned broth, you should omit the salt and test for seasoning later) and a couple turns of freshly ground black pepper. Bring to a boil, reduce the heat to a simmer, cover and gently cook for about 45 minutes over a stovetop or crockpot or until the lentils are tender.
Remove the garlic and herb bundle and add the apple cider vinegar. Test for seasoning.
Remove about 1 ½ cups of the soup and puree using an immersion blender. Add back into the pot to create a creamier consistency.
Serve with a sprinkling of freshly chopped parsley, if you like and crusty bread.