Begin by roasting the beets.
Tie onion, carrot, celery, bay leaf, and thyme sprigs together in cheesecloth. Place chicken stock in a medium saucepan and bring to a boil. Stir in the lentils, season with salt and pepper, and reduce the heat to medium-low and cook until the lentils are tender, about 20 minutes. Drain well and discard the cheesecloth bag of aromatics.
Heat the oil in a large saute pan over medium heat. Add the bacon and cook until lightly golden brown. Remove the bacon with a slotted spoon to a dish lined with paper towels. Add the garlic and carrot to the pan and cook until soft, about 5 minutes. Add the cooked lentils and bacon to the pan and stir to combine. Stir in the chopped thyme and 1 tablespoon of sherry vinegar. Season with salt and pepper, to taste. Keep warm.
To assemble, place the frisee in a large bowl, toss with half of the vinaigrette, and season with salt and pepper. Arrange the roasted beets around the outside of 4 dinner plates. Divide the greens among each plate in the center. Top the frisee with some of the warm lentils and place a slice of the goat cheese on top. Drizzle each salad with the remaining vinaigrette. Serve with slices of French bread.
Preheat oven to 375 degrees F.
Coat beets lightly with oil. Wrap beets in aluminum foil, place on a baking sheet, and roast in the oven until cooked through, approximately 45 to 60 minutes. Remove from the oven, let cool for 10 minutes, and then peel and slice into 1/4-inch thick slices.
Whisk together the vinegar, mustard, thyme and salt and pepper in a small bowl. Slowly drizzle in the oil and whisk until emulsified.