Sweet Potato and Sausage Gratin with Thyme

Preheat oven to 400 ° F. Cook sausages in heavy large skillet over medium-high heat until brown and cooked through, crumbling with back of spoon, about 6 minutes. Using slotted spoon, place sausage in small bowl. Pour off any drippings in skillet. Add broth, wine, apricot nectar and 2 tablespoons butter to same skillet; boil until liquid is reduced to 1 1/2 cups, about 15 minutes. Pour into another bowl. Melt 2 tablespoons butter in same skillet over medium heat. Add leeks and sauté until tender, about 10 minutes. (Can be prepared 1 day ahead. Cover and refrigerate sausage, broth mixture and leeks separately.) Arrange half of potatoes in 13 x 9x2-inch glass baking dish. Sprinkle with half of thyme. Season with salt and pepper. Sprinkle with 1/3 cup cheese. Top with half of leeks and all of sausage. Cover with remaining potatoes. Sprinkle with remaining thyme. Season with salt and pepper. Cover with remaining leeks. Top with remaining cheese. Pour broth over. Cover and bake 30 minutes. Uncover and bake until potatoes are tender and liquid thickens, about 15 minutes. Cool slightly.