Trim hard parts from mushrooms and discard. Rinse mushrooms quickly to remove grit; cut caps into slices and stems into small chunks. Combine in a medium skillet with 1 clove garlic (crushed and peeled), thyme, olive oil and some salt and pepper. Cook over sufficiently low heat so that mixture bubbles but mushrooms do not brown.
Continue to cook, stirring infrequently, until they are quite shrunken in size and appear firm, 30 to 45 minutes. (At this point, you can pause for several hours before resuming cooking.)
If you are using porcinis, soak them in hot water to cover for a few minutes so that they soften.
Set a large pot of water to boil for pasta and salt it. Mince remaining garlic. Turn heat under oil to medium, and add garlic to the portobellos along with the drained porcinis. Cook, stirring occasionally, until garlic begins to colour. Add tomatoes and raise heat to medium high. Cook, stirring occasionally, until they break up and become saucy, about 15 minutes.
Meanwhile, cook pasta until it is tender but not mushy. Taste sauce and adjust the seasoning. When pasta is done, drain it, toss with sauce, top with parmesan, and serve.