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1
Adjust the oven rack to the middle position and preheat the oven to 350F.
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2
Peel away and discard the outer layer of each leek and trim and discard the hairy roots, leaving the very ends of the roots attached.
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3
One at a time, place the leeks on the cutting board and, with the white end pointing away from you, remove the dark green part of the leek, cutting it at an angle and rolling the leek a quarter-turn to repeat the same cut.
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4
Continue rolling and cutting in this manner until the tapered pale green ends resemble a sharpened pencil.
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5
Discard the dark green trimmings.
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6
Cut the leeks in half lengthwise, rinse to remove the fine grit, and pat dry.
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7
Place the leeks in a shallow baking dish just large enough to hold them in a single layer, such as a 9 by 13-inch Pyrex dish.
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8
Drizzle the leeks with 1/4 cup of the olive oil, sprinkle them with the salt and several turns of pepper, and toss to coat the leeks with the seasonings.
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9
Arrange the leeks cut side up in a single layer in the baking dish and pour in the stock, adding more if necessary for it to come three-fourths of the way up the sides of the leeks.
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10
Drizzle the remaining 1/2 cup of olive oil over the leeks, and lay the lemon slices and scatter the thyme sprigs over them.
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11
If you have industrial-strength plastic wrap, which does not melt in the oven, cover the pan tightly with the plastic.
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12
In any case, cover the pan tightly with foil and place the leeks in the oven for 45 minutes.
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13
Remove the leeks from the oven and increase the temperature to 400F.
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14
Remove and discard the foil, and plastic if you used it; remove and discard the lemon slices and thyme sprigs, and return the leeks to the oven until they are glazed and golden brown and the liquid has evaporated, about 1 hour 15 minutes.
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15
Cool to room temperature and serve, or wrap them in a single layer with plastic wrap, and refrigerate them for up to two days.
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16
Bring the leeks to room temperature before serving them.
