Put the lentils into a small pot, cover with twice their volume of cold water, then add cinnamon, thyme, garlic and onion. Bring to the boil then lower the heat and simmer until the lentils are just tender, about 15 minutes. Drain well and let cool, discarding the spices.
Combine the lentils in a large bowl with all the remaining ingredients except for the lettuce. Season with salt and pepper to taste and toss to combine.
Stand mixture for 10 minutes to allow flavours to develop then serve on a bed of lettuce leaves.