Position oven rack at the lowest level; preheat to 450°F. 

2, Coat a 9 1/2-inch, deep-dish pie pan with cooking spray. Line the bottom with parchment paper or foil and lightly coat with cooking spray.

2, Heat oil in a large nonstick skillet over medium-high heat. Add leeks and thyme. Cook, stirring often, until tender, about 5 minutes. (If necessary, add 1 to 2 tablespoons water to prevent scorching.)

Season with 1/8 teaspoon salt and pepper.

Arrange half the sweet potato slices, slightly overlapping, in the prepared pie pan and season with a little of the remaining salt and pepper. 

Spread one-third of the leeks over the top. 

Arrange half the potato slices over the leeks and season with salt and pepper. Top with another third of the leeks.

Layer the remaining sweet potatoes, leeks and potatoes in the same manner. Cover the pan tightly with foil.

Bake the torte until the vegetables are tender, about 45 minutes. 

Run a knife around the edge of the torte to loosen it. Invert onto a serving plate. Remove paper or foil and serve.

Tip: To clean leeks: Trim and discard coarse green tops. Split leeks lengthwise with a sharp knife, beginning about 1 inch from the root end and cutting toward the green end. Leave root end attached. Swish leeks repeatedly in a basin of cold water to remove grit. Alternatively, trim roots and ragged tops. Slice leeks and place in plenty of water, then drain. Repeat a few times. The slices do not absorb water or lose flavor and the process is faster.