TO MAKE THE VINAIGRETTE: In a small bowl, whisk together the vinegar, salt, shallots, and thyme. Add the olive oil in a slow stream, whisking to emulsify. Season with pepper and more salt, if needed.
TO MAKE THE SALAD: Heat the oven to 400 degrees F If you choose to make your own beets: Rub the beets with the olive oil, put them on a baking sheet, and bake until fork-tender, 60 to 90 minute When cooled, peel off the outer skin and slice the beets into 1/4-inch rounds.
Cut the baguette into 12 slices about 1/4 inch thick. Toast under the broiler until light golden brown on one side; turn them over and lightly brown the other side. Cool. With a hot, wet knife, cut the goat cheese into 12 slices. Put a round of cheese on each slice of baguette. Sprinkle with the herbes de Provence, thyme, and pepper and set aside.
When ready to serve the salad, heat the oven to 400 degrees F. Put the croutons in the oven until the cheese softens, about 5 minute Meanwhile, arrange a few slices of beets on salad plates. Season lightly with salt and pepper. In a large bowl, toss the greens, haricots verts, and olives with enough vinaigrette to just moisten them. Divide the mixture among the salad plates. Put a warm crouton on either side and serve.