In a large Dutch oven or heavy-bottomed pot, cook the chopped bacon over medium heat until crispy, about 6-8 minutes. Remove bacon with a slotted spoon and set aside, leaving the fat in the pot.
Add the onion, carrots, and celery to the bacon fat and cook until softened, about 5-7 minutes. Stir in the garlic and cook for another minute until fragrant.
Add the lentils, broth, diced tomatoes (with their juices), bay leaves, and thyme. Bring to a boil, then reduce heat to low and simmer, partially covered, for 30-35 minutes until lentils are tender.
Stir in the chopped kale or spinach and cook for 3-5 minutes until wilted. Remove bay leaves.
Stir in the red wine vinegar and most of the crispy bacon (reserve some for garnish). Season generously with salt and pepper to taste.
Let the stew rest for 5-10 minutes before serving. Top each bowl with reserved bacon pieces. Refrigerate leftovers for up to 4 days—it only gets better!