In a separate pot, bring water to a boil and cook pasta according to package directions.
In the pressure cooker, over medium heat, toast the walnut halves until fragrant. Stir frequently so they do not burn. Remove walnuts and set aside.
Remove any visible grit for the porcini mushrooms, add them to the cooker along with the 1 cup of water.
Lock the lid in place, bring to HIGH pressure, and cook for 7 minutes. Release the pressure.
Drain the porcini mushrooms reserving the liquid, it will be about 1/3 of a cup, again check for any grit. Rinse the porcinis to remove any additional grit and chop.
In the cooker, heat the olive oil and lightly sauté the sliced fresh mushrooms and porcini. Add the white wine and reserved liquid. Lock the lid in place, bring to HIGH pressure, and cook for an additional 5 minutes. Release the pressure.
Slightly reduce the remaining liquid in the pot. Remove from heat and stir in the grated cheese and cream. Season to taste with salt and pepper. Add the prepared pasta noodles and reserved toasted walnuts and toss. Serve with a drizzle of truffle oil if desired.