Chop about two-thirds of the bacon into 1/4-inch pieces; set aside. Wrap the thyme, bay leaf, peppercorns and garlic in a piece of cheesecloth; tie closed with kitchen twine. Combine the cheesecloth bundle, the remaining slab bacon, the lentils and 1/2 teaspoon salt in a medium saucepan and cover with cold water by about 2 inches. Bring to a boil, then reduce the heat to medium and simmer until the lentils are tender, about 25 minutes. Drain, discarding the cheesecloth bundle and bacon; let cool.
Meanwhile, cook the chopped bacon in a medium nonstick skillet over medium heat, stirring occasionally, until golden brown and just crisp, about 10 minutes. Drain on paper towels; set aside.
Make the dressing: Whisk the mustard and vinegar in a large bowl. Slowly whisk in the olive oil until emulsified. Toss the beets with about 1 tablespoon of the dressing in a small bowl; season with salt and pepper.
Assemble the salad: Add the lettuce, endive, shallot, parsley, chives and the prepared lentils and bacon to the bowl with the dressing; season with 1/4 teaspoon salt and a few grinds of pepper and toss. Top each serving with some beets.