Soak the porcini in 1 cup very hot water until soft, 10 to 15 minutes. Fish out the porcini and slowly pour the soaking liquid into the turkey broth, leaving any sediment from the mushrooms behind; you should have about 4 cups combined liquid. Chop the porcini finely.
After transferring the turkey to a cutting board, pour the drippings from the roasting pan into a heatproof measuring cup. Let the fat rise to the top and then spoon 1/4 cup of the fat back into the roasting pan. Set the roasting pan over two burners on medium heat. Sprinkle the flour into the pan and use a flat whisk or wooden spoon to combine it with the fat. Cook for about 2 minutes.
Slowly pour 1/2 cup of the broth into the pan while whisking vigorously to disperse the flour. The liquid should thicken quickly and become gluey. As soon as it thickens, whisk in another 1/2 cup broth. Repeat until the gravy starts to look more like a smooth sauce; then whisk in the remaining broth and bring to a simmer. Add the thyme and simmer for 5 minutes. Strain the gravy through a medium sieve and stir in the chopped porcini and lemon juice. Season to taste with salt, pepper, and more lemon juice.
nutrition information (per serving):
per 1/4 cup;