In a large pan over medium heat, heat enough oil to coat the bottom of the pan along with about 1 tsbp of butter until the butter has melted and the oil is hot. Meanwhile, bring a large pot of water to a boil. When it boils, which will be when you're about mid way through making the puree, add 1 pound of spaghetti or pasta noodle of your choice and cook according to package directions, about 8-10 minutes, in most cases.
Add the shallots to the hot oil and butter and simmer until they are soft and light blonde in color, about 5 minutes. Add the garlic and simmer, 1-2 minutes, until just soft and lightly golden.
Add the sliced mushrooms to the pan and stir to coat them in the shallots, garlic, oil and butter. Add a bit more olive oil if the pan is very dry (no more than a drizzle, the mushrooms will release plenty of their own liquid as they cook).
Add a healthy amount of salt and pepper to the mushroom mixture. I never measure this exactly. You can always add more at the end, if necessary.
Simmer the mixture for 10-15 minutes, until everything is deeply browned.
Transfer the mixture to a blender and blend until everything is smoothly pureed, about 30 seconds. It will look like gray baby food. It's the unfortunate reality of a mushroom puree, but it looks better on the noodles and it tastes amazing!
Transfer the mixture back to the hot pan. Add the cream, nutmeg and bay leaf and stir to combine. Warm the puree over low heat until the pasta has finished cooking, giving the bay a few minutes to impart its flavor. Add about 1/4 cup of the starchy pasta water to the sauce to thin it out a bit, and then add the pasta noodles directly to the sauce and stir until the noodles are totally covered in the puree, adding more pasta water if necessary to help the puree distribute among the noodles. Taste for seasoning and add salt and/or pepper, if necessary.
Serve with parsley, additional sauteed mushrooms, crispy fried shallots, toasted nuts, parmesan cheese and/or any other garnish you like. Enjoy!