Root Vegetable Gratin

Name: Root Vegetable Gratin
Ingredients: 6 tablespoons extra virgin olive oil (divided use); 3/4 pound medium beets (peeled and cut into 1/8-inch slices); 3/4 pound orange sweet potatoes (peeled and cut into 1/8-inch slices); 3/4 pound large parsnips (peeled and cut into 1/8-inch slices); 3/4 cup vegetable or chicken stock (See Cook's Tips); 3/4 cup finely chopped shallots; 3 cloves garlic (minced); 2 tablespoons chopped fresh thyme leaves (divided use); 1 teaspoon kosher salt (more to taste); 1/2 teaspoon pepper; 1/4 cup grated parmesan cheese; 1 tablespoon thinly sliced chives