Warm the oil or melt the butter in a large Dutch oven or soup pot over low heat.
Toss in the onions, celery (including the leaves), and carrot; stir the vegetables until they glisten, then cover the pot and cook, stirring often, until they soften but don't color, about 10 minutes.
Add in the broth, lentils, citrus peel, peppercorns, coriander, clove, and ginger; increase heat and bring to a boil.
Decrease heat so that the broth just simmers, put the cover on the pot, and gently simmer until the lentils are so soft you can mash them with the back of a spoon, 60-90 minutes.
After the soup has simmered for about 45 minutes, season it with salt and pepper.
Working in batches, puree the soup-spices, celery leaves, orange peel, and all-in a blender or food processor; or use an immersion blender.
Wipe out the pot and pour the soup back into it, and reheat.
Ladle the soup into plates or bowls; garnish each with a dollop of yogurt, and sprinkle with bacon, if desired.