Preheat oven to 400 degrees. Bring a large pot of water to boil over high heat, add a generous amount of salt to taste and the rigatoni and cook, stirring occasionally, for 12 minutes, or until the pasta is almost al dente. Strain the pasta in a colander, rinse it under cold running water, and toss with 1 tablespoon of oil.
With a slotted spoon remove the soaked mushrooms from the liquid and strain the liquid through a fine sieve lined with dampened cheesecloth or a coffee filter into a bowl. Reserve. If the Porcini mushrooms are still dirty, rinse them and pat dry. Trim the stems and coarsely chop the mushrooms.
In a 2 1/2 quart ovenproof casserole set over moderately low heat, melt the butter and add the onions, and salt and pepper to taste and cook, stirring occasionally, for 20 minutes, or until the onions are golden brown and tender. Transfer the onions to a plate.
Add the pancetta to the casserole, increase the heat to moderate and cook, stirring occasionally, for 15 minutes, or until the pancetta begins to brown. With a slotted spoon, transfer the pancetta to the plate with the onions. Add the Portobello mushrooms to the casserole, season with salt and pepper and cook the mushrooms, stirring occasionally, for 10 minutes, or until tender. Add the garlic and cook, stirring for 1 minute. Add the chopped Porcini mushrooms, strained mushroom juice, onions, pancetta, 1/2 cup of the Parmesan, and the heavy cream to the casserole and simmer, stirring occasionally, for 10 minutes, or until mushroom flavor is pronounced in the cream. Add the pasta, spinach, thyme, salt, and pepper and gently toss the mixture with the remaining Parmesan. Transfer the casserole to the oven and bake for 20 minutes, or until cream is bubbling.
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