Deep-hued lentils and roasted beets make a delicious combination.
Preheat oven to 350 degrees. In a medium bowl, toss beets with 2 tablespoons oil and 1 1/2 teaspoons salt. Arrange on a rimmed baking sheet; pour 1/4 cup water into pan. Cover with foil; roast until beets are easily pierced with the tip of a knife, 45 to 60 minutes. Remove from oven; let cool. Peel and cut into 1/2-inch cubes.
Meanwhile, combine lentils, garlic, and bay leaves in a 6-quart saucepan; add enough cold water to cover by about 3 inches. Bring to a boil over medium-high heat. Reduce heat to medium-low; simmer, stirring occasionally, until lentils are tender but not mushy, 10 to 20 minutes. Stir in remaining 2 teaspoons salt, and cook about 5 minutes more. Drain in a colander; let cool.
Make vinaigrette: In a small bowl, combine shallot, mustard, and vinegar; let stand 15 minutes. Slowly whisk in remaining 1/4 cup oil in a steady stream.
Transfer lentils to a large serving bowl. Pour vinaigrette over lentils, and add tarragon; toss well to combine. Toss in beets. Season with salt and pepper, and serve, or cover with plastic wrap and place in refrigerator up to 4 hours; bring to room temperature before serving.
To prevent lentils from becoming too soft as they cool, spread them on a baking sheet.