Mix the two flours, the salt and porcini powder in a large bowl and mix to combine. Make a well in the center of the flour. Pour the water into the well in the flour. Mix with a fork until shaggy, then mix with your hands until the dough comes together. Knead on a floured surface for 4 to 6 minutes, or until the dough is elastic, and not sticky. Wrap in plastic wrap, and set aside for at least 1 hour, and up to a day in the fridge.
Brown the porcini in the butter, then add the garlic. Sprinkle some salt over the pan. Saute everything until the garlic turns golden, then pour in the cream and the thyme. Simmer gently until the porcini are soft, about 5 minutes. Toss the thyme sprigs and scrape everything into a blender. Buzz into a puree. Taste for salt again and add some if needed.
Set yourself up a large area to work. Cut the dough into four pieces. Cover the rest with the plastic wrap. Roll the dough out until it is a long, thin sheet -- but not all the way to the final setting; I go to No. 6 on my Atlas machine, where No. 9 is the thinnest. Cut the sheet in half. Place about a teaspoon of filling at the center of one sheet, about 2 inches apart. Pat the filling down into a flattish disk. Lay the other sheet on top of the first one, and press it down around the filling, leaving no air pockets. Use a pasta cutter or a knife to cut the ravioli out. Set each one on a baking sheet dusted with cornmeal.
To finish, get a large pot of water boiling, and salt it well. In a large saute pan, melt the butter over high heat. Add the mushrooms to the pan, cut side down if you've halved them. Brown them well on that side, then flip and cook on the other. At this point, move the ravioli to the boiling water, about 10 at a time. Boil for 1 minute after they float to the surface. Move them to the pan and toss with the fresh mushrooms and butter. Add the sage and freshly ground black pepper. Spritz with a touch of lemon juice and serve.