In a 2 cup (1/2L) jug add the porcini mushrooms and pour in hot water to the 1-1/2 cup (360ml) level. Set the jug aside for about 1 hour to let the mushrooms rehydrate.
Meanwhile, with a lightly dampened kitchen towel (paper or cloth), gently wipe any dirt from the crimini mushrooms. Coarsely chop or slice the mushrooms into 1/2" (12mm) pieces.
Gather and prepare the remaining ingredients.
When the porcinis have softened and cooled, gently lift them out of the soaking liquid and squeeze out as much water as you can. Take care not to disturb the grit at the bottom of the jug. Coarsely chop the porcinis and add to the chopped criminis.
Carefully pour the porcini soaking liquid into a small pot, again taking care to leave behind the grit on the bottom of the jug. Set the pot over medium high heat and bring to a boil, reducing the liquid to about 1/4 cup (60ml). It should have a slightly syrupy consistency.
Measure 2 qt (2L) of water into a large pot with 1 TBS Kosher salt or 1 1/2 Tsp of sea salt and bring to a boil (See note below). Have ready a colander set in a large bowl in the sink.
Heat the oil in a 12" (30cm) skillet over medium high heat and sauté the shallots, garlic and crushed red pepper flakes for about a minute. Add the crimini and porcini mushrooms with 1 Tsp of Kosher salt (or 1/2 Tsp sea salt) and 1/4 Tsp of pepper.
Once the mushroom are going, add the pasta to the pot of boiling water to cook, stopping 2 minutes shy of the recommended cooking time on the package. (Set your timer once the pasta comes to a boil.)
Cook the mushrooms, tossing occasionally, for 8-10 minutes. The crimini mushrooms will release their water which will almost completely evaporate by the time the pasta is done.
Add the tomato paste, rosemary, reduced porcini soaking liquid and 1/2 cup (120ml) of pasta water to the mushrooms and give it a quick stir.
Drain the pasta in the colander you have ready in the sink. Add the pasta to the mushrooms along with 1/2 cup of pasta water.
Toss the pasta with the mushrooms for 2-3 minutes, adding pasta water by the 1/2 cup until the pasta is cooked al dente. (Continue cooking the pasta and adding water if you prefer your pasta more done.) By the time the pasta is cooked there should be a tablespoon or two of thick, starchy water left in the skillet.
Off heat toss the pasta another minute or two, adding a dribble of pasta water as necessary if the pan gets too dry. This lets the pan cool off a little before adding the parmesan cheese.
Add the butter and parmesan cheese, tossing to create a creamy sauce. Add a tablespoon or two of pasta water as necessary to get the sauce consistency you like.
Finally, stir in the pepitas before serving on warm plates with a sprinkle of cheese and a drizzle of olive oil. Or, if you are feeling decadent you can drizzle on a little truffle oil instead.