Place the mushrooms in the base of a food processor and pulse ~8-10 times until very finely chopped. Alternatively, you can do this by hand. Remove and place in a bowl, then add the onions. Again, pulse ~8-10 times until very finely chopped. Remove.
Heat the olive oil in a large skillet over medium heat. Add the mushrooms and onion and cook until very well browned and the mushrooms have released most of their liquid. This should take about 20 minutes; stirring often. Deglaze with the soy sauce, scraping up any browned bits.
While the mushrooms are cooking, make the sauce. Add the sun-dried tomatoes, garlic, sage, basil, nutmeg, chili powder, garlic powder, pumpkin puree, tomato sauce, and broth. Let simmer for ~20 minutes, stirring often, then transfer to a blender or use an immersion blender to puree until creamy and smooth.
While the sauce is cooking, bring a large pot of salted water to a boil. Add the pasta and cook until just al dente, then drain reserving ~1/4 cup pasta water.
Once the mushrooms are very brown, add the pumpkin-tomato sauce and stir together, then add in the cream and cooked pasta along with a tablespoon of pasta water. Toss, toss, toss, coating each pasta strain with the sauce until very well coated. Season to taste, adding salt/pepper/red pepper flakes as needed, and serve!