Put the dried porcini in a bowl, cover with water, and soak for at least 15 minutes.
Fill a pot for the pasta with at least 6quarts water, place over high heat, and bring to a boil.
Peel and finely chop the onion. Put it in a 12-inch skillet with the butter over medium-high heat. Sauté, stirring occasionally, until the onion turns a rich golden color, about 5 minutes.
While the onion is sautéing, brush any dirt off the fresh mushrooms and thinly slice them.
When the porcini have finished soaking, lift them out of the water and squeeze the excess water back into the bowl; do not discard the water. Rinse the porcini under running water, then coarsely chop them. Add the porcini to the onions along with the soaking water. Be aware that there may be some sand at the bottom of the bowl, so pour carefully or strain through a paper towel. Cook until the liquid is evaporated, then add the fresh mushrooms. Season with salt and pepper and cook until all the liquid the mushrooms release is evaporated, 10 to 15 minutes.
While the mushrooms are cooking, peel the tomatoes and coarsely chop. When the mushrooms are done, add the tomatoes, season with salt, and cook until they are no longer watery, about 10 minutes.
Add about 2tablespoons salt to the boiling water, put in the penne, and stir well. Cook until al dente.
Add the cream to the sauce and cook until it is thick enough to coat a spoon. When the pasta is done, drain it well, toss with the sauce, and serve at once.
You can make the sauce through step 6 up to 2 days ahead. Store, covered, in the refrigerator. Reheat before proceeding.