Step 1Heat the oven to 250 degrees. Heat the olive oil in a large Dutch oven or ovenproof pot and saute the onion over medium heat until it becomes translucent and begins to soften, about 3 minutes. Add the bacon and cook until it softens, about 5 minutes. Remove the pot from the heat and remove any excess oil by patting the bacon and onion with a paper towel.
Step 2Return the pot to the heat and add half the greens. This will fill the pot, but as you cook, stirring frequently, the leaves will wilt and shrink. When there is enough room, add the remaining greens and the chicken stock. Stir to mix evenly.
Step 3Cover the pot and place it in the oven to cook until the greens are well stewed and deeply fragrant, about 1 1/2 hours.
Step 4Add the lentils, stir, cover and return the pot to the oven until the lentils are tender but still a little chewy, about 40 to 45 minutes.
Step 5Season to taste with salt and pepper and stir in 1 tablespoon of balsamic vinegar. Taste and add a little more vinegar if necessary. Serve immediately.