Cook bacon until just crispy, over medium flame in the bottom of a Dutch oven or large stockpot. Transfer to paper towel-lined plate and reserve. (If you’re making it vegetarian, skip this step and heat olive oil in the Dutch oven over medium flame.)
Add onion, carrots and celery to either the bacon fat or olive oil in the pot. Season lightly with salt and generously with pepper, and cook for about 5 minutes, stirring frequently. You don’t want to brown the the vegetables, just sweat them—reduce heat, if needed.
Clear a space in the middle of the pan, and add the thyme and garlic. Cook, stirring constantly, about 45 seconds. Add the lentils to the pot and stir everything together.
Add the chicken or vegetable broth and bring to a boil over medium-high flame, stirring occasionally. Reduce heat, cover pot and simmer until lentils are just tender, 20 to 30 minutes. (The older dried pulses and beans are, the longer they take to cook; start sampling at 20 minutes.)
Stir cannellini beans and kale into the pot, adding water, if it’s cooked down too much. I actually added a cup of water. Cook until kale is tender and beans are heated through, 5 minutes or a little longer. Taste and adjust seasoning with salt.
Divide among four soup bowls. Top with crumbled bacon—or sprinkle with Parmesan if you’re making the vegetarian version. Serve.