Preheat oven to 450 degrees F (230 degrees C). Coat beets lightly with oil and roast for approximately 45 minutes, or until tender. Allow to cool thoroughly, then peel and dice.
For the dressing, place lemon, vinegar, honey, mustard, and thyme in a blender. While blender is running, gradually add 1/2 cup of oil. Season to taste with salt and pepper. Place spring lettuce mix in a salad bowl, pour desired amount of dressing over greens, and toss to coat.
Rinse endive, tear off whole leaves, and pat dry. Arrange 3 leaves on each plate. Divide dressed salad greens among them, and top with diced beets and feta cheese.