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1
Preheat oven to 450u00b0F with rack in middle.
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2
Wash leek halves and pat dry.
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3
Toss leek, carrots, garlic, bell peppers, plum tomatoes, fennel, and onions with oil, then spread in a 17- by 14-inch roasting pan and roast, stirring occasionally, until well browned and tender, 1 1/4 to 1 1/2 hours.
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4
Pour boiling water over porcini in a bowl and soak until softened, 10 to 20 minutes.
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5
Meanwhile, wrap celery, parsley stems, thyme sprig, peppercorns, and bay leaves in a piece of cheesecloth and tie with string.
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6
Lift out porcini, squeezing excess liquid back into bowl, and rinse to remove any grit. Pour soaking liquid through a sieve lined with a dampened paper towel into another bowl.
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7
Transfer roasted vegetables to a 6- to 8-quart pot and add wine to roasting pan, then deglaze pan by boiling, scraping up brown bits, 1 to 2 minutes. Add wine to vegetables in pot along with water (4 quarts), porcini and soaking liquid, bouquet garni, sun-dried tomatoes, and 2 teaspoons salt. Bring to a boil, then simmer, uncovered, until stock is reduced to about 6 cups, about 2 hours. Pour through a fine-mesh sieve into a large bowl, pressing firmly on and then discarding solids.
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8
Melt butter in a 3-quart heavy saucepan over medium heat and whisk in flour, then cook roux, whisking, 3 minutes. Add stock in a stream, whisking constantly to prevent lumps, then bring to a boil, whisking. Reduce heat and simmer, whisking occasionally, until slightly thickened, about 10 minutes. Season with salt and pepper.
